I don't understand what you mean by all this. Obviously your phone number is tied to your Signal account and Signal knows that linkage at all times - no special action is necessary for them to know it. Signal does not provide anonymity beyond what a phone number provides, and isn't designed to.

Signal's design is "what if WhatsApp wasn't evil" which makes it good for the average user. If you actually want Session or SimpleX, which sacrifice usability for even more privacy, then you know where to find those services.

There's all sorts of things any service provider could do to sleuth out who its users are. The implication of this OFAC determination is that A/I didn't need to sleuth anything out; they are accused of knowingly and deliberately assisting the PKK.