The difference is that OpenID is not a 'well known startup' but an industry standard and that Apple obviously implemented the well-known protocol 'OpenID Connect' but added some bugs and didn't want to mention that their technology could be compatible with the existing one.
And I don't want to downplay the innovative additions Apple made to the whole thing (browser integration etc.), but it doesn't make an honest impression if you don't mention that your revolutionary product is a based on a well-known standard.
