Paul seemed to assume at first they were already believers, yet why would Paul go on to say,. “John indeed baptized with a baptism of repentance, saying to the people that they should believe on Him who would come after him, that is, on Christ Jesus" if they truly were already believers? Also, why the need to baptize them again if they truly were already believers?
Well, Apollos was mighty in the scriptures, but needed to be expounded the right way by Aquilla and Priscilla. In Acts 18:27.. he came to Achaia and was received by disciples who had believed through grace.
He knew only the baptism of John. John's baptism was the in between from OT to NT. After John, baptism became the ordinance of the New Testament local church.. So Paul would need to re-baptise because these disciples were not baptised under the right authority of a New Testament church. There was nothing wrong with the baptism John was administering.. but there came a point where future believers would need to be baptised in a different NT authority.
But again.. John was baptising with those he baptised professing belief in Jesus as God and Saviour. If these disciples in Acts 19 were going by that baptism.. they would have also believed. This is confirmed when it says they had 'believed thru grace'.
The part where I could be wrong on this.. is if the disciples in Acts 19 and the disciples in Acts 18 are different groups. But in Acts 19.. it is assumed they had believed. Their belief isn't the question... the baptism is.