Yes, Tether and any firm that is in the business of minting and burning virtual currency would qualify as an administrator. Indeed it is registered with FinCEN and state in their user agreement, “Tether monitors for and assesses suspicious or sanctionable transactions under applicable AML, Anti-Corruption, and Economic Sanctions Laws, as well as undertakes mandatory reporting to FinCEN, OFAC, FIA, and international regulators.”
That said, you’re absolutely right that Tether (and presumably Libra) can be held by non-KYC’d public key-pairs in between on- and off-ramps. What is not clear to me is how far their BSA obligations would reach. For example, if Libra Assoc is an administrator, then it will be their obligation to “know their customers.” But who are its customers? Is it only the ‘resellers’ of Libra for whom the Assoc mints and destroys? Or is it any user of the network it runs? I think regulators could easily interpret it as the latter, even if it’s impossible to comply with that. Also, imagine OFAC adds some Libra addresses to the sanctions list (as they have done with bitcoin addresses), I wonder if the Libra Assoc wouldn’t have to blacklist those addresses via all validators.
Hope that helps!