If the man was Jewish, then it the burden would have been to mention that he was Jewish. Instead it said he was Ethiopian. Reading Isaiah isn't limited to Jews nor the definition of Jewish, and eunuchs were prevented form becoming proper Jews regardless.
Slow down there...
Isaiah 56:1 Thus saith the LORD, Keep ye judgment, and do justice: for my salvation is near to come, and my righteousness to be revealed. 56:2 Blessed is the man that doeth this, and the son of man that layeth hold on it; that keepeth the sabbath from polluting it, and keepeth his hand from doing any evil. 56:3 Neither let the son of the stranger, that hath joined himself to the LORD, speak, saying, The LORD hath utterly separated me from his people: neither let the eunuch say, Behold, I am a dry tree. 56:4 For thus saith the LORD unto the eunuchs that keep my sabbaths, and choose the things that please me, and take hold of my covenant; 56:5 Even unto them will I give in mine house and within my walls a place and a name better than of sons and of daughters: I will give them an everlasting name, that shall not be cut off. 56:6 Also the sons of the stranger, that join themselves to the LORD, to serve him, and to love the name of the LORD, to be his servants, every one that keepeth the sabbath from polluting it, and taketh hold of my covenant; 56:7 Even them will I bring to my holy mountain, and make them joyful in my house of prayer: their burnt offerings and their sacrifices shall be accepted upon mine altar; for mine house shall be called an house of prayer for all people.
To this very day, the Law and the Prophets are very important among the Ethiopians.
By the way, during the Greek conquest, Greek became the common language of many countries. Resulting in a need for the translation of the OT that was still prevalent in the 1st Century - the Septuagint, a Greek translation of the OT.
Thus, it would not have been a problem for anyone so interested to seek out, towards obtaining, a personal copy of one Book of the OT or another.
Not to mention, that Ethiopian is described as "a man of Ethiopia, an eunuch of great authority under Candace queen of the Ethiopians, who had the charge of all her treasure, and had come to Jerusalem for to worship, had come to Jerusalem for to worship," obviously, per the above concerning eunuchs, described in his copy of Isaiah 56.
A note - passages like the above make obvious that that Ethiopian is merely a type of the Gentile Nations when Israel's Kingdom is Restored, one day - they will not be required to submit to circumcision, nor to those aspects of the Law meant to identify ONLY Israelites as God's Kingdom of Priests OVER said Gentile Nations.
Deuteronomy 4:1 Now therefore hearken, O Israel, unto the statutes and unto the judgments, which I teach you, for to do them, that ye may live, and go in and possess the land which the LORD God of your fathers giveth you.
4:5 Behold, I have taught you statutes and judgments, even as the LORD my God commanded me, that ye should do so in the land whither ye go to possess it. 4:6 Keep therefore and do them; for this is your wisdom and your understanding in the sight of the nations, which shall hear all these statutes, and say, Surely this great nation is a wise and understanding people. 4:7 For what nation is there so great, who hath God so nigh unto them, as the LORD our God is in all things that we call upon him for? 4:8 And what nation is there so great, that hath statutes and judgments so righteous as all this law, which I set before you this day?
Not surprisingly, this Two-Fold People during that time (Israel as a Kingdom of Priests OVER the Gentiles, one day) described by Isaiah in the above, is often described by him - here is another example...
Isaiah 66:7 Before she travailed, she brought forth; before her pain came, she was delivered of a man child. 66:8 Who hath heard such a thing? who hath seen such things? Shall the earth be made to bring forth in one day? or shall a nation be born at once? for as soon as Zion travailed, she brought forth her children. 66:9 Shall I bring to the birth, and not cause to bring forth? saith the LORD: shall I cause to bring forth, and shut the womb? saith thy God. 66:10 Rejoice ye with Jerusalem, and be glad with her, all ye that love her: rejoice for joy with her, all ye that mourn for her: 66:11 That ye may suck, and be satisfied with the breasts of her consolations; that ye may milk out, and be delighted with the abundance of her glory. 66:12 For thus saith the LORD, Behold, I will extend peace to her like a river, and the glory of the Gentiles like a flowing stream: then shall ye suck, ye shall be borne upon her sides, and be dandled upon her knees.
66:19 And I will set a sign among them, and I will send those that escape of them unto the nations, to Tarshish, Pul, and Lud, that draw the bow, to Tubal, and Javan, to the isles afar off, that have not heard my fame, neither have seen my glory; and they shall declare my glory among the Gentiles. 66:20 And they shall bring all your brethren for an offering unto the LORD out of all nations upon horses, and in chariots, and in litters, and upon mules, and upon swift beasts, to my holy mountain Jerusalem, saith the LORD, as the children of Israel bring an offering in a clean vessel into the house of the LORD. 66:21 And I will also take of them for priests and for Levites, saith the LORD. 66:22 For as the new heavens and the new earth, which I will make, shall remain before me, saith the LORD, so shall your seed and your name remain. 66:23 And it shall come to pass, that from one new moon to another, and from one sabbath to another, shall all flesh come to worship before me, saith the LORD.
In short...
Nehemiah 8:8 So they read in the book in the law of God distinctly, and gave the sense, and caused them to understand the reading.
Try that some time.
Romans 5:6-8.