If you will read the end of Acts 18, you will see why Paul does not teach them Jesus.
Acts 18:24 Now a Jew named Apollos, a native of Alexandria, came to Ephesus. He was an eloquent man, competent in the Scriptures. 25 He had been instructed in the way of the Lord. And being fervent in spirit, he spoke and taught accurately the things concerning Jesus, though he knew only the baptism of John. 26 He began to speak boldly in the synagogue, but when Priscilla and Aquila heard him, they took him aside and explained to him the way of God more accurately. 27 And when he wished to cross to Achaia, the brothers encouraged him and wrote to the disciples to welcome him. When he arrived, he greatly helped those who through grace had believed, 28 for he powerfully refuted the Jews in public, showing by the Scriptures that the Christ was Jesus.
We see Apollos taught Jesus accurately but only knew John's baptism. That's why there were believers but had not been baptized into Jesus' name.
V.27 Apollos went to Achaia which is where Corinth is located. While Apollos was there Paul rolls into Ephesus.
19:1 And it happened that while Apollos was at Corinth, Paul passed through the inland[a] country and came to Ephesus. There he found some disciples. 2 And he said to them, “Did you receive the Holy Spirit when you believed?” And they said, “No, we have not even heard that there is a Holy Spirit.” 3 And he said, “Into what then were you baptized?” They said, “Into John's baptism.” 4 And Paul said, “John baptized with the baptism of repentance, telling the people to believe in the one who was to come after him, that is, Jesus.” 5 On hearing this, they were baptized in the name of the Lord Jesus. 6 And when Paul had laid his hands on them, the Holy Spirit came on them, and they began speaking in tongues and prophesying.
If they were disciples of John, they would not know Jesus died for their sins and we would see Paul telling them that.