Yup, Thijs is correct. It‘s just a credit card without a credit line. Nothing revolutionary or hard to understand, really. At some other banks you can also get the same type of card (I have even seen such a card as a savings account alternative once…)
That means you still get interest calculated on your actual higher balance, something that would never happen with an actual debit card. It also means you have better chargeback protections in case of a company going bankrupt or similar, because you‘re actually paying with the bank‘s money, not directly with yours. And last but not least, the acceptance might also differ, as some places take only credit cards, which includes the bunq Mastercards.
Yes, the billing that is shown to you is similar to a debit card. That is one improvement that bunq did because many people prefer it that way and it makes it a lot harder to accumulate any debt and you won‘t have to pay your bank any interest fees, ever. Really it‘s a card unlike any other, a credit card bunq-style 🌈