next generation of chip is back to AM4 or some AM5

Threadripper isn't going to use AM4 or whatever its successor is. It's always been using a different socket since the first gen (TR4, the Threadripper equivalent to AM4).
It's basically a double Ryzen socket, to overly simplify it.

Inside of the Threadrippers is the same basic structure as the Ryzen line, with some glue added to put multiple ones in one package. So this is likely using the Ryzen 3XXX structure, just like Threadripper 2 was basically some Ryzen 2XXX glued together.

As for the socket change, Threadripper isn't really a server CPU (That'd be the EPYC line) and workstations aren't really expected to keep the same socket and just upgrade like a Ryzen (It'd be nice, though)