pl0x MasterCard Debit (which it is
It's officially a Mastercard Credit. Just bunq does not offer you a credit line. But in the background, whenever you make a purchase, that money is collected and your monthly credit card bill automatically gets paid on the 1st of every month. You don't have to worry about it like with a debit card, because bunq does it all for you and does not allow you to spend any money twice, but the money still generates interest on your account every month until the 1st, so technically it's still yours, just with no way to access it.
Also from a technical standpoint, the cards are credit cards. It's printed "Credit" on them and when you read them out with a card reader, they present as credit cards, not as debit cards. Therefore, acceptance is the same as a credit card (which is the point of all of this).
In the end, of course feel free to call them any way you want, there's not really an established term for cards that work like this. "Secured credit card" comes close, but it's different, as these normally work with a deposit and you still technically go into debt every month that you then have to pay. Just that if you're not able to pay, your deposit can be used, so you're not actually gonna carry the debt any further. Basically a kind of "secured credit card" that works even simpler and with even less risk.