It means the same thing the Lord Jesus said here when He was already there:
"For the Son of man shall come in the glory of his Father with his angels; and then he shall reward every man according to his works" (Mt.16:27).
Of course you just ignore this passage which I previously quoted that shows that the kingdom will not be at hand until the Lord Jesus returns:
"And then shall they see the Son of man coming in a cloud with power and great glory. And when these things begin to come to pass, then look up, and lift up your heads; for your redemption draweth nigh. And he spake to them a parable; Behold the fig tree, and all the trees; When they now shoot forth, ye see and know of your own selves that summer is now nigh at hand. So likewise ye, when ye see these things come to pass, know ye that the kingdom of God is nigh at hand" (Lk.21:27-31).
And if the kingdom was in place at Matthew 12:28 then we must stand reason on its head and believe that all of the people on the earth were born again. After all, the Lord Jesus said:
"Jesus answered and said unto him, Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God. Nicodemus saith unto him, How can a man be born when he is old? can he enter the second time into his mother's womb, and be born? Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God" (Jn.3:3-5).
What the Lord Jesus was saying at Matthew 12:28 is that since He casts out demons by the spirit of God then the kingdom of God has come in the sense that the King is right in front of them.
Or perhaps you are willing to argue that the kingdom of God actually came upon the Pharisees to whom He was addressing.
Nothing there proves that anything which I previously said about Matthew 10:23 is in error.