But he said the iCloud subscription was not due or past due. which means Apple did not try to charge his card for the iCloud service and then locked his account because they couldn't

The iCloud subscription was not past due. [1]

1. https://twitter.com/dcurtis/status/1366485756546539520

It can't be just that. iCloud storage and Apple Music were two of the subscriptions I was paying for, to get that sweet 3% cash back.

It's possible that the card was maxed out and payments for the subscription failed, maybe? I caught mine before that happened.

Edit: a sibling comment makes it clear that that isn't what happened: https://news.ycombinator.com/item?id=26308468