As you say, permission information only uses 5bytes and 1 bit (setuid etc are not permission bits, they are really the 'type' of the object and so are is some ways more closely related to IF_REG etc). Being that precise in the article would have been excessive I think. It is still true that the permissions were stored in 6 bytes. It is just that some room was left over for file type as well.
POSIX ACLs may well be mostly ignored, but ACLs are still the only direction being explored for making the permission model more complete. My point was simply that they have a storage cost which gets worse quickly, but worse than that it has a serious usability cost.
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