Deleted Characters Being Reviewed

They actually don't have a creation date in the character database unless they've added one.
Just the logout date.

Which is probably why they used the date the Cygnus skill was acquired (this would be in the Quest table). But does not explain why they used April 7th, give or take a day.

Not sure how they're doing the UA link either but I doubt it's in the character table, more likely a separate table like spouses.

...and performing a Join is way beyond their abilities.
Although, as has been pointed out elsewhere, a normal adventurer does not need a KoC on the account in order to get the skill. They can just do the Lunar Dew quest at level 70. So perhaps the test for a KoC is unnecessary.

If I were Nexon, with their renowned delicacy, I'd just go ahead and ban any account that got the skill within the past 2-3 weeks or however long it was, and tell the users to submit a ticket with a preset title if they think they were banned wrongfully. Then set up an automated filter to handle those tickets, and a procedure to check, for example, if the character has any quests completed significantly earlier (indicating it's an old normal adventurer), or still has its KoC link, or the user can provide screenshot proof of having had a KoC at the time the UA was created. Obviously if the char is a Cannoneer or has unreleased hair or eyes, the appeal would be rejected out of hand.