So I’ve always used debit cards, and know other people who do as well, and it’s never been an issue. The cards all have either a visa or mastercard logo on them. When I input the debit card in seller central it’s even identified as a debit card on the page you enter it on.
I have had issues in the past where a card was rejected (and you get the message to enter a credit card to regain access to the account), but it wasn’t on the basis that it was a debit card. It was an issue of the billing zip code not matching and once that was updated it got accepted.
Logically speaking, the reason why Amazon wants a card on file is they have a payment method that they can charge for any fees incurred, and a debit card serves that purpose. One argument I can think of is someone might not have money in their linked bank account, but that same argument applies with a credit card that’s almost maxed out.
I think people complaining that their debit card isn’t working have some other issue, and it’s not that Amazon strictly wants a credit card. They might have some billing address mismatch (this is actually tricky when it comes to debit cards, sometimes the AVS billing address doesn’t update properly if you update the address associated with the bank account itself), or if they’re using a non-US bank account, their debit card may not allow international/different currency charges.