Shouldn't be any question. You took the Attack action as long as you already made one attack with your action (since, you know, you're committed at that point).
So Shield Master with this ruling works as many in that recent thread thought it should: Make the first attack of your action, but after that you can shove before making your other attack(s).
That is one interpretation. Others rule that the action must be completed (all attacks) before you've "taken" it. I go the other extreme of saying that it doesn't even require an attack - simply saying "I'm attacking the ___" is good enough.
It's been debated ad nauseam over on other threads.