It should be possible to independently verify Signal's attestation, but I don't know if anyone has done it. Before you go and say "what's even the point then",

It should be possible to independently verify Signal's attestation, but I don't know if anyone has done it. Before you go and say "what's even the point then", the point is that this gives Signal plausible deniability for when the TLAs show up asking for user info.