Hacker News Reader

Why not use an existing open standard like TOTP instead of creating a custom app? Seems less of a hassle to me.

They actually answered that, something with the user experience of using two apps and TOTP not meeting requirements for the highest eIDAS level. See interview here (in Dutch): https://www.security.nl/posting/701749/Security_NL+spreekt+m...

Sorry to keep posting this same sort of comment here, but: does that say anything about U2F or FIDO2?

No; just TOTP.