> I believe it had something to do with TypedDict instances being instances of dict at runtime, but I can't actually reconstruct the reason.

Hm that may be true.
My limited low-level Python knowledge leads me to believe that this could also be done using __new__ but I also read that most magic methods get called as type(Foo).__magic__(bar, ...) so that might not be possible.
(However also no methods can be declared on a TypedDict class so that might not be a problem?)

> Maybe it's written up in PEP 589, but I suspect not (I skimmed and couldn't find it).

I read it completely and could not find anything

> If you ask on typing-sig maybe David Foster (who contributed the initial idea and implementation) remembers.

I asked [here on typing-sig](https://mail.python.org/archives/list/[email protected]/thread/RNFWPRLHTUTZES2FDSSMY472JFGMD4EW/) but did not yet get any responses.