Not sure what happened. I only had minolta TCs and minolta lenses that worked with TCs. But I think minolta was also "3rd party" as far as lens and TC combinations was concerned.
if Sony limits their TCs to only be used with 1st party lenses i can get on board with that too as long as they allow sigma to make their own TCs for e-mount. But looks like they won't allow the latter option too.
Tbh I'm not fully sure why Sony has issues with TC in general. Even a-mount Sony TCs can't be used with Sony a-mount lenses via. the LA-EA adapters. But if get one of those chipped LA-EA4 adapters you can actually use TCs. So once again they are limiting it while technically it's possible to support it.
Canon EF and Nikon F TCs works fine when adapted to RF or Z mount respectively. So I think Sony has some kind of a TC phobia