Whereas you can (and I believe always could*) create an apple ID with any old email address.
*Maybe this delinked situation only came about when they added the App Store to OS X, and figured they'd make less money if they require existing Mac users to get a new email account in order to buy programs in the manner which would grant them a cut.
Apple has a list of all the email addresses for its sole IDs, but it doesn't control them, and having one deleted doesn't nessisarilly affect the other.
Google and custom domain email have always been delinked from this perspective. You could create a Google account with a custom domain and then point the domain elsewhere or lose control of it, and you'd still retain conto of the account.
Basically, the required example essentially theoretical at this point - maybe it works for employers at companies that also happen to provide SSO services. So if you work at Facebook, Google, Apple, or github and have a me@FGAG.etc.com email dress, and you signed into slack through the SSO that affiliated with your company and the company email, but later don't work there and you've had your work account access revoked, you won't be able to use that SSO to sign into slack. That's what they mean by directory control or whatever.
In contrast, if you sign up to github with your work email account, unless it's a managed device managed by your work, your work doesn't actually control the account. They just vouched for your affiliation at sign up when you verified your email. So if you use a github SSO to sign up for a service that 'verifies' your work email address from github during the process, that won't change when you leave and the company revoked access to the email. Github SSO, in this case, isn't verifying you have an email account @company.com. They are verifying you once did, or at least once had, access to it. This is what they mean by the non-directory whatever.
Similar with Apple, if you were signing in with an @icloud.com, it's pretty good proof, but if you have an Apple ID with a third-party e-mail it's not proof of current control of that e-mail.
That's my guess.
Also, is it impossible to have a Google account with a non-gmail address? The original poster seemed to be saying that Google _is_ a directory SSO and Apple _is not_ categorically. But if you can have a Google account without a Gmail-ran email account, wouldn't Google have the same vulnerability?
Google accounts have the exact same issue so I don't understand the distinction made by the OP though.
You can have your own domain if it is a workspace account
This can be done with an account that you once had control over but don't anymore, like if you leave an employer.
You can't send mail from it, but many apps will take having a google account with a given email is proof of ownership, or an @example.com email address is proof that you are an employee of Example Inc. when they are a customer of the app and have a tenant set up.