My guess is that M3 Macs were supposed to come this Autumn (likely MBA, 13" MBP and iMac). It's possible that with TSMC's yield issues on the first gen 3 nm line, all the capacity had to be directed to the A17 Pro and the M3 launch got pushed back to 2024 when the 2nd gen 3 nm line is churning out the chips. If that is the case, Apple may have decided to push a previously unplanned M2 iMac out of the door (and then delay an M3 iMac until the 3 nm supply has stabilised). As pointed out, there is no reason that an M2 iMac couldn't have launched alongside the M2 Mac Mini earlier this year.