Is playing a card as hidden considered playing a card?

No, playing a card as hidden is not considered playing a card.

According to Rule 737.1.c.1, "Hide is not a subset of Play." Hiding a card is a "Discretionary Action" distinct from the action of playing a card.

As clarified in the RiftJudge FAQ (ID 8500):

• Hiding a card does NOT count as playing it. When you pay the cost to place a card facedown at a battlefield, you are performing the action of "Hide," not "Play." Consequently, this does not trigger "When you play" abilities or count toward Legion requirements.
• Playing a card from hidden DOES count as playing it. When you flip the card over and resolve its effects on a subsequent turn, you are officially playing that card, which opens a chain (Rule 737.1.c.3).