Member reading time is indeed the main (if not sole factor for earnings).

As for people finding your article through other platforms, if they are a Medium member and are signed in, it does not matter where they found your article.

However, obviously many of those coming from external searches/platforms will not be medium members. These reads, while likely boosting your article’s overall rank will not contribute to earnings (unless they subsequently sign up for medium in the next 30 days).