It's an old comment.
iirc it's a documented feature now - FEAT_E2H0, https://support.arm.com/documentation/109697/2025_12/Feature...
And it was retroactively defined to be allowed starting from Armv8.0.
Apple designs pre-date the ID register bit for it being a thing so it takes a quirk there however.
This just means that Apple is a big enough player for ARM to retroactively amend the standard. Does not mean that what Apple did was not a violation of then-standard tho.