The unitarians in Ephesus had not been baptized by John
When Paul later returned to Ephesus, he found some ignorant non-trinitarians there. It is obvious that these 'ignoramuses' had been altogether uninfluenced by (and were quite probably even unaware of) the incisive and instructive Spirit-filled Scripture-preaching of that eloquent and learned Christian, Reverend Apollos. Indeed, those ignorant non-trinitarians seem to have arrived in Ephesus — only after Apollos had departed thence for Corinth.88
It had been some twenty-five years since the death of that great trinitarian, John the Baptizer. The whole Christian Church Universal knew that John had baptized especially the Lord Jesus — the central Person of the Holy Trinity, and the Only-begotten and Spirit-anointed Son of God the Father. Yet, most surprisingly, a group of ignorant non-trinitarians was now making the claim to Paul that its approximately twelve members had priorly been initiated "into John's baptism." Still more remarkably, each of them was apparently altogether ignorant about the very existence of the Holy Spirit — and perhaps even of the Lord Jesus Himself.
Now John the Baptizer, while baptizing people, had always pointed his baptizees (and prospective baptizees) away from himself — and toward the coming Messiah (Jesus Christ). In so doing, John had always told them how this Spirit-anointed One would soon Himself endue them — not with water, but indeed with His Holy Spirit.89
The unitarians in Ephesus, however, not even alleged they had received their 'baptism' by or from John himself. They only claimed — and even that claim itself is suspect! — to have been initiated: "into John's baptism."90 Indeed, they frankly admitted to Paul that they themselves had never even heard whether there is a Holy Spirit.91
This "John's baptism" which these unitarians here alleged they had received, was therefore quite devoid of the Holy Spirit. Even if it really had been given to these unitarians — this "John's baptism" had obviously not been administered to them by the trinitarian John himself. For he had been a Spirit-filled person (ever since his conception and also from his mother's womb). Indeed, even after growing up, John still testified about the Holy Spirit. He did so during his Spirit-filled preaching, and also while baptizing. Luke 1:15-17,44-44,80; 3:3-16f; Matthew 3:2-11f; Acts 10:37f; 13:24f.
So this 'Spirit-less' rite referred to in Acts 19:3b by the unitarians — was certainly not Johannine. Nevertheless, an important question must still be asked. What indeed had happened to those unitarians — before they met Paul in Ephesus?
It seems almost certain that, earlier, other unitarians — people like the syncretistic Essenes — had misled the men Paul now met in Ephesus. Even those earlier unitarians had perhaps never even seen or heard the trinitarian preacher John himself. They had certainly quite misunderstood his teachings — probably encountering them by hearsay, or even third-hand. They had then — perhaps quite inadvertently — gone and started yet one more judaistic sect. Compare Acts 22:16 & 23:6-9 and First Corinthians 15:29.
The men Paul met in Ephesus were apparently members of just such a sect. For it seems that after John's death, syncretizing unitarians had themselves started initiating people "into John's baptism." By this, they probably meant they were purporting to initiate 'in the name of John' or perhaps even 'into the name of John' — neither of which John himself would ever have done.
Clearly, this 'Spirit-less' rite was certainly not the Christian baptism John himself had administered. For even while baptizing with water, John had urged his baptizees soon thereafter to become the disciples of the Spirit-baptizing One — Jesus Himself. Indeed, John's disciples were never even supposed to be 'rebaptized' — neither by Jesus, nor by anyone else.
It is quite inconceivable that anyone could have been baptized by John himself without, right then, hearing about the Holy Spirit from the Spirit-filled and Spirit-witnessing John himself. However, the 'Spirit-less' rite referred to in Acts 19:3b — was quite devoid of the Spirit.
Consequently, it could not have been Johannine. Indeed, that Spirit-less water-rite — previously received, if at all, by the Spirit-less men who now met Paul in Ephesus — could itself have started probably only after John's own death.