I share most HN'ers priors about this administration and its motives and am commenting positively, not normatively, and only about the specific claim the Treasury Department made.

Signal doesn't knowingly and deliberately provide services to any organization, let alone one formally designated by the US and the EU. The whole point of Signal is to eliminate all pretenses of the service knowing who its users are or what they're doing. To the extent that's also true of Autistici Inventati, the OFAC designation is invalid.