I'm actually thinking that leaving the Update tab in App Store open for a long time won't do it. I'm thinking that closing it, waiting a while, and opening it again is what was needed.
I'm thinking this because when Update is opened, it makes a request to a server. If that request fails to arrive, fails to execute, or the results don't appear, I'm thinking that additional requests aren't sent. (I'm thinking this due to my limited Apple product experience, past behavior I've seen.) I'm thinking that you have to manually send any additional requests by re-opening Updates.
FWIW, the whole 2 days my Updates wouldn't open, my Purchased DID open. Purchased recognized my Apple ID and behaved properly while Updates did not. That speaks to me of a different server/cluster receiving the request. I didn't try Featured, Top Charts, or Genius during the period when Updates wouldn't open but Purchased did open, sorry.