Can you activate Sun Disc's ability the turn you play it, counting itself for its Legion requirement?

Ruling: Yes, you can activate Sun Disc's ability the turn you play it because Sun Disc itself counts as a card played that turn, satisfying the Legion requirement.

Sequence:

• When you play Sun Disc, it counts as playing a card this turn
• Sun Disc's Legion ability becomes active immediately
• You can exhaust Sun Disc that same turn to ready the next unit you play
• On future turns, you must play another card first before activating Sun Disc's ability

Nuances:

• Sun Disc's ability is NOT an on-play effect, so it follows the Legion rule that checks "if you have played a Main Deck card this turn" rather than "another card"
• You cannot activate Sun Disc without having Legion active - the ability doesn't exist unless you've played a card that turn
• The reminder text saying "another" card is technically incorrect for this type of Legion ability