Maybe they were gifted the gift card - Or they default to purchasing their Apple stuff using gift cards only because they don't want their credit card data saved on Apples side. Who knows?

Because it was a 500 dollar transaction and the card they purchased was an apple-branded product in a major retailer.

It was a 500 dollar transaction because this guy is a pro developer in Apple's ecosystem and apparently uses a 6TB plan for both personal and professional storage.

The Trigger: The only recent activity on my account was a recent attempt to redeem a $500 Apple Gift Card to pay for my 6TB iCloud+ storage plan. The code failed. The vendor suggested that the card number was likely compromised and agreed to reissue it. Shortly after, my account was locked.
    An Apple Support representative suggested that this was the cause of the issue: indicating that something was likely untoward about this card.
    The card was purchased from a major brick-and-mortar retailer (Australians, think Woolworths scale; Americans, think Walmart scale), so if I cannot rely on the provenance of that, and have no recourse, what am I meant to do? We have even sent the receipt, indicating the card’s serial number and purchase location to Apple.

Much as I do think mixing pro and personal accounts is a mistake, as a person who has to pay several major corpos for subscription plans for professional software that include cloud storage, I admit I get it. Receiving spam about how full your free personal Google Drive is kinda sucks extra if you are already paying a bunch for an enterprise account with a bunch of storage on the side.