They are an alternative to 2FA. Which means they aren't 2FA. If they were 2FA, they wouldn't be an alternative to 2FA. They'd just be 2FA.

Anyway, passkeys and FIDO broadly aren't the same thing. You can read the definition of passkeys at https://fidoalliance.org/passkeys/ or look at any of the marketing, which invariably talks about how great it is that you don't have to futz with passwords anymore.

FIDO credentials in general can obviously also be used as second factors. This is baked into the name of the original standard: U2F, Universal 2nd Factor. The specific point of passkeys though is that they're the single factor.

Many do what you describe, probably because some manager somewhere needs to tick some checkbox.

But GitHub, specifically, allows you to sign in with a passkey. On the sign-in page, there's a "sign in with passkey" link. It activates my 1Password extension, asking if I want to use my passkey. I say yes, and I'm in, I don't type anything. This also works the same way with my YubiKey.