There was in fact a division within Jewish scholars about whether or not there was an afterlife. I’m not sure where the people OP is referring to get the idea that there was “no idea of resurrection in first-century Judaism” since the point is made in the gospels that the Sadducees disagreed with the Pharisees and also with Jesus over precisely this issue (Mark 12:18-27). The belief in an afterlife develops gradually in Judaism - there is some indication that King David believed in it but before that there is little evidence of such a belief.
Having heard Christian apologists like William Lane Craig make the point before, I don’t think that they’re claiming there was no idea of a resurrection. The christian apologists I’ve heard usually say that Jews believed in a “general resurrection” at the end of time, but had no belief in any kind of individual resurrection
prior to the general resurrection. Therefore, the apologists argue, the apostles would not have been inclined to believe such a tale of an individual resurrection, such as that of Jesus, therefore the apostles would never have believed in such a resurrection unless they had truly seen it with their own eyes.
The John the Baptist quote, given by the OP, is meant to illustrate that in fact some Jews
did come to a belief in an individual resurrection, prior to the general resurrection at the end of time, even without seeing any actual evidence of it, thus debunking that particular point that these apologists have tried to make (which is, of course, only one of the many points that are made concerning the resurrection of Jesus, which may or may not be valid, but this particular point, that Jews could never have thought up an individual resurrection, is untrue even according to Christian scriptures, and should be discarded by apologists).
V