You are correct in that a Connection is a link between a local account and a provider account. However, I was not comfortable forcing capture of the providerAccountId at the level of all AbstractServiceProvider implementations, given it's not needed by the ServiceProvider framework to obtain a Service API.
So we've removed that in the meantime while we think through how we want to handle the case where additional provider profile information is needed to support user stories such as provider sign-in, rendering of connected profiles, etc. This discussion on the forum has been quite helpful in that regard.
Core Spring Development Team