It’s not clear to me how GitHub would enforce the “we don’t use enterprise repos” stuff alongside “we will use free tier copilot for training”.

A user can be a contributor to a private repository, but not have that repository owner organisation’s license to use copilot. They can still use their personal free tier copilot on that repository.

How can enterprises be confident that their IP isn’t being absorbed into the GH models in that scenario?

We do not train on the contents from any paid organization’s repos, regardless of whether a user is working in that repo with a Copilot Free, Pro, or Pro+ subscription. If a user’s GitHub account is a member of or outside collaborator with a paid organization, we exclude their interaction data from model training.