Whether or not creditors can tell that they are just AU's, I dont know.
Yes, they can, if the creditor reports it properly. The data format that's essentially universally used by creditors and the credit bureaus has a field for type of relationship the individual has to the account. There's a code for individual, one for joint, one co-signer, one for primary with co-signer, and one for AU. (Maybe a couple others; probably one for 'he's dead'.) The creditor can do this either by attaching an extension segment to the the record for the main creditor, or by making a separate record for the AU. (They can do both, too, and put the primary account holder on the associated consumer field of the AU. I don't think anyone would do that on purpose.) I don't know what AMEX is doing these days. They used to do it correctly, except they'd sometimes report a code that had ambiguous meaning (which isn't in the current data specification). I bet they largely still do the correct reporting.
Even if the creditor has no other information except a name, they can report it as an associated consumer at the primary address. That may, or may not, be enough to get the account on someone's credit report. (It should be, if the name is correct, and they have other accounts at that address.)
Some credit issuers entirely discount AU accounts, others don't.
Also the amex limit on the AU is per billing cycle, not calculated like a credit limit. (They may take payments made by the AU into account; you'd have to ask about that.)