Are you sure that is the C terminal and not RC it is common on thermostats to have an R and RC jumpered if there is not a second transformer in the condenser.
hunted down the manual for my old tstat. and you are correct.
it’s RH jumpered to RC. I just didn’t see the R in both instances because it was hidden by the fat red jumper.
I guess it’ll mean using the C-Wire adapter. which means going up in the attic and wiring up both units. upstairs/downstairs.
good thing it’s not summer in texas.