Was 'overseers' simply a generic, non-titular use of the term?
Imo yes, at the simplest level of reading, and it is quite natural in the context of the shepherd/flock metaphor which uses the related verbs episkeptomai / episkopeô quite often (LXX, Jeremiah 23:2; Ezekiel 34:11f; Zechariah 11:16 etc.; cf.1 Peter 5:2).
And at the same time it certainly alludes to episkopos as a current/emerging title.
It is not either/or, if the very point of the Acts passage in its own setting (just as the pseudepigraphy of the Pastorals) is to groundthe current development (the church gradually uniting around their one "bishop" in the face of threatening centrifugal "heresies," cf. the "wolves" of Acts 20, which are relevant to the author's situation, not to the narrative of Acts per se) into Paul's words as a past reference -- hence necessarily to be set in the earlier, still known, pattern of collective leadership (presbuteroi). Not so different from Titus 1:5 from this perspective.
This kind of "double entendre" is essential to the narrative construction of Acts and the Gospels, which deal with current issues from the standpoint of the author(s) through past narratives which have to be kept plausible as such.