How are they not true debit cards? They are issued by banks, directly debiting from an account. Sure, internally they piggyback on infrastructure originally intended for credit cards, but that's an implementation detail.

Maestro, given you mention it elsewhere, doesn't necessarily have longer payment card numbers than most credit cards (which are mostly 16, except for Amex which is mostly 15): it merely has a number made up of a number of different prefixes, and the final section is determined by the issuing bank… and several of these are variable length.

I think the grandparent may be from a country in which a separate debit network, operated jointly by the nation's banks, exists. In such countries banks have typically issued cards using these networks and they may be seen as "true debit cards". MC/Visa are then often used interchangeably with "credit card", even in instances where they are debit cards on the Visa/MC network. (I'm not arguing this is correct usage of terminology.)

Anecdata: I'm from Germany, which is one such country. I've recently had a conversation with someone who had a "credit card" that charges directly to their bank account.