Yes in that there have been rumors there will be a new iMac in the 23" form factor and it could be replacing the 21.5" model.
RDNA1 availability was after the 2019 iMac refresh, was it not?
NAVI 10 XL, XT were released on July 7, 2019, way after the 2019 iMac. At this time, Polaris was the only thing AMD had to offer.