"Total number of places across all Guides: 300"

These limits wouldn't be an issue if Google Maps API actually allowed fetching saved list features.

I have over 2,000 places saved in Guides in the Apple Maps app. Not sure what that 300 place limit is referring to, but I haven't had any issues.

For Google Maps, They have an “official” limit of 500 where anything beyond that is not guaranteed. In practice, the current limit is 3,000.

I’m wondering if Apple Maps is doing something similar where they set a low official limit that they can walk back on / to in the future, as to avoid legal responsibility

AFAIK the API doesn’t even let you log in as your Google user account.