This only tells us what they do by default and without gag-order. They could still be forced to log specific users and are barred from telling us by legal request.

Furthermore, it is known from Telegram disclosures that the FBI has been approaching staff from messenger companies with the offer of quite a lot of money to act as moles inside these companies.

As long as Signal is a centralized service with servers in the US, neither problem can be solved and that makes Signal inherently unsafe to use.