When Ava Achiever's attack ability allows you to play a hidden cost card from hand, does that count as playing it 'from hidden' for the purposes of Teemo's ability?

Ruling: No. Playing a card with the hidden keyword is not the same as playing it from face down (from hidden). Ava plays the card from hand, not from the hidden zone.

Nuances:

• Teemo, Strategist was errataed to remove the "played from hidden" trigger entirely, making this interaction moot for that version
• If Teemo is played as a defender through Ava's ability, he would get his effect (for versions that trigger when defending)