Well, these are disciples, and they did believe.
Acts 19:1-3
And it happened, while Apollos was at Corinth, that Paul, having passed through the upper regions, came to Ephesus. And finding some disciples he said to them, "Did you receive the Holy Spirit when you believed?" So they said to him, "We have not so much as heard whether there is a Holy Spirit." And he said to them, "Into what then were you baptized?" So they said, "Into John's baptism."
They knew the message of John the Baptist and believed it.
when Paul said "when you believed" he wasn't deceived - they believed at least this much:
John 1:29-34
The next day John saw Jesus coming toward him, and said, "Behold! The Lamb of God who takes away the sin of the world! This is He of whom I said, 'After me comes a Man who is preferred before me, for He was before me.' I did not know Him; but that He should be revealed to Israel, therefore I came baptizing with water." And John bore witness, saying, "I saw the Spirit descending from heaven like a dove, and He remained upon Him. I did not know Him, but He who sent me to baptize with water said to me, 'Upon whom you see the Spirit descending, and remaining on Him, this is He who baptizes with the Holy Spirit.' And I have seen and testified that this is the Son of God."
so they believe Jesus is the Christ, the Son of God, the Lamb, come to take away the sin of the world. their trust is in Him, in Christ, in God Who sent Him.
Paul did not need to preach this to them, and the record is that he didn't.
What He did add to them, was the knowledge of what had happened at Pentecost, and imparted to them a spiritual gift, laying his hands on them.
They were not unbelievers but believers.
IMO this is not their having become saved or sealed. this is their being filled with the Spirit in power.