As Google seems to explicitly allow that, does it actually matter if I download opengapps and flash it myself versus getting it from APK mirror or Aptiode?
Presumably, if you bought a phone that originally had the Play Store and then flash it with Lineage, Google has licensed that phone already to run Google Apps. The preloading agreement with the manufacturer, which is what this license scheme is designed to protect, is still working as intended, so Google has no reason to intervene.
Google's behavior would also generally lead one to believe they are not concerned about what enthusiasts/tech pros do with their phones, as that's a niche quantity of users who don't really negatively impact their bottom line. They're chiefly concerned with ensuring manufacturers are only selling devices with their apps installed and set as default.
I agree they are chiefly concerned with manufacturers. However, if a user bought a phone from a device manufacturer that did not license it, from that article, it seems like they can still license it themselves. However, that opens up the idea that if that becomes more widespread, than they would lock down this "loophole".
I mainly bring it up as if an app is "free" (as in beer), does it really matter where you get it from (assuming it is untampered with)? I know they can forbid redistribution in their ToS, but is that actually enforceable legally?
Yes. Illegal redistribution violates the owner's copyright (in its absolute literal sense). While there may be no actual damages, a country might allow for statutory damages to be sought after. For example, the US allows for such statutory damages:
Google's behavior would also generally lead one to believe they are not concerned about what enthusiasts/tech pros do with their phones, as that's a niche quantity of users who don't really negatively impact their bottom line. They're chiefly concerned with ensuring manufacturers are only selling devices with their apps installed and set as default.