Thinking that the only things available to the US security state from Signal are the things Signal claim to save is, I feel, naive in the extreme.
On the upside, them leaking, acknowledging, or unambiguously using that in a way that is traceable is going to be a massive upset, and you're probably not that important.
But Apple, Google, Signal, are all US organisations with servers in the US operating under US wiretapping laws.