Though OS packaging may frequently obscure this because OS packagers break up the language distribution, Idle is part of the language distribution, so (unless you expand the distribution to include core packages outside of stdlib, as with Ruby’s gemification effort) it has to be built with stdlib alone. Tkinter, therefore, needs to be part of stdlib.

Depends, do you consider IDLE part of the standard library? (it's separated in many distributions, but documented in the standard library documentation, so it's a bit mixed, but it's probably okay to not consider it part of the library, but just the standard distribution)