I suspect Paul's opponents were saying that Christ and everyone else are translated immediately elsewhere upon death (heaven/paradise?) so there is no need to speak of a resurrection especially not a future one or a physical one. Paul is cleverly framing that argument to be non-sensical but it only works if there is a future physical resurrection in the first place. I suspect Paul here is being forced to affirm a physical resurrection when he might not have originally spoke of it (the 'being caught up in the clouds' stuff) to fit in with a prevailing view that he feels he must also affirm"If there is no resurrection of the dead, then Christ has not been raised"
So am I saying there was no prior belief then that Christ was resurrected physically until Paul?
Well, it is possible to imagine talk of a spiritual non-physical encounter with the risen Christ sure... perhaps Paul made it a requirement for that to have been a physical resurrection. It all gets a bit confusing ... is a bright light and vision 'physical'? Are spiritual things taking place in the material world 'physical'? I just think here Paul is driving a wedge in, making his gospel the only acceptable one as he generally does.
In other words Paul is doing battle with an excess of immanence, but it's very obvious immanence forms a great part of his own gospel so my suggestion is immanence is the prior belief, and this certainly connects into the history of Jewish belief generally which does not have originally a concept of resurrection - thus there is an argument Paul's opponent are holding onto a more traditional belief here in this area. Hence all the ambiguity in the gospel accounts over whether Jesus is physically resurrected or not (mistaken identities, lights, visions, over the top confirmations it is physical, Mark's short ending etc, the famous comma, this day you will be with me in paradise). Paul sometimes is driving a square peg into a round hole, his opponents had the luxury of possessing a round peg