Perhaps you're right that they couldn't be compelled by law to reveal it, then! However, I can still find people on Signal using their phone number, by design. If they can do that, surely there is sufficient information, and appropriate means, for US state-side signals intelligence to do so, too. I don't think Signal self-hosts their infrastructure, so it wouldn't be much of a challenge considering it's a priority target.

Now, whether FBI and friends would be determined to use PII obtained in this way to that end—is a point of contention, but why take the chance?

Better yet, don't expose your PII to third parties in the first place.