The fact that apple would reuse an existing chip doesn’t negate the fact that it really doesn’t take much RAM at all to manage the additional storage. It takes almost no extra RAM, in fact.
It's weird, but they did it before with the 2018 model. I believe the 2018 model had 4gb and 6gb on the largest model (1Tb?). That additional 2gb supposedly for managing storage and the ProCreate devs confirmed it wasn't accessible by applications.
The memory difference doesn't seem to be on the overview or the store/purchase page. It's only on the detailed tech specs page (under CPU), which is also rather strange. You'd think it would be on the purchase page!
Then again, I believe the iPhone / Pro have different amounts of RAM and Apple don't advertise that difference either 😖