Mike Mearls is simply wrong.
Read the book, play the game, learn from your experiences, ignore the tweets.
He's wrong when he says that an ability check requires an action. It's unclear whether he means an action in terms of the turn structure, or an action in the informal sense of putting forth effort to do something, but either way there are obvious counterexamples:
- Initiative is an ability check to determine how quickly you notice a situation, and is prior to anyone taking any action at all.
- A Grapple requires an opposed Athletics check, meaning both the attacker and the defender make checkschecks; it replaces a single attack by the attacker, and costs the defender no action/reaction at all.
- The examples in the rules for Acrobatics checks include "stay upright on a rocking ship's deck". You're literally making a check to just stand there.
- Talking generally does not require an action, and may result in a Persuasion or Deception or other check to determine what effect your words have. This may (as in your situation here) be an opposed check, so even the character who's not talking has to make an ability check.
He's also wrong about automatically succeeding when a DC is below your passive skill. As described in the PHB, passive skill doesn't apply to normal ability checks at all. It applies to passive checks, which are a different kind of check made under different circumstances.
In general, if you are allowed to roll for a check, you don't also get to apply your passive skill to that check.