You might think that the
function would tell you whether a SID is well-known, but it doesn't.
Rather,
it tells you whether a SID exactly matches the well-known SID
you specified.
For example, you can ask,
"Is this the Authenticated Users SID?"
or
"Is this the Everyone SID?"
But you can't ask,
"Is this any type of well-known SID?"
I guess ...