I suspect they didn't mention it because they see the lock-in to their product as a feature. Mentioning OpenID might give the impression that Apple would be interested in interoperability (to some degree).

they didn't mention it because if they did, they would need to be a member of the openid foundation and pay the 15k$.

There's no requirement to be to a member of the OpenID Foundation to describe your implementation as OpenID; the trademark rules are here: https://openid.net/intellectual-property/trademark-license/

The main requirement therein I would say is that you can't use OpenID to describe an implementation unless it implements the mandatory-to-implement parts of the relevant protocol specification (and all the specifications are 100% free; e.g. https://openid.net/specs/openid-connect-core-1_0.html ). That's more about interoperability than anything else.