Using standards has some advantages like not having to test dozens of authentication systems/clients doing basically the same thing. If you would have read the document you would have found that you can't use standard openID clients due some issues with Apple's implementation. Some of the issues have security implications.

Not really...

For example, the lack of a discovery document means that a lot of the essential configuration cannot be discovered programmatically.

The lack of a UserInfo endpoint means that applications which are used to using that will need to get the user info out of the id_token.

edit: not “no expense report”; i actually meant “you don’t have to ask anyone first (but will have to file a report about it later)”, which is not quite the same thing.

You're probably joking, but in case you're not this isn't right. If a company had a policy like that they would have difficulty demonstrating to the IRS that all their expenses really were for their business, and they would have a hard time protecting themself from embezzlement. The company needs to be keeping a receipt and a business justification for the expense.

$15k is something like 0.000006% of Apple's cash on hand.

$15,000 would be between a week and just over a month of compensation.

See https://www.levels.fyi/salary/Apple/

I don't think that's right. Perhaps you meant 18 weeks?

It's still irrelevant, as it's not a something an individual is paying. The parent comment merely scales Apple's cash on hand to numbers we can understand.