That really doesn't affect the legality (or, frankly, morality) here for many reasons, but the most obvious is that the word "unofficial" is certainly not going to be used in all of the discussions about the software as it simply isn't actually in the name, which is clearly only "signald".
> or, frankly, morality
Sorry, are you saying it is immoral to name a daemon that interacts with Signal 'signald'?
First off, morals are subjective, but, also, yes: if you're going to have a different project, it should have a different name. Otherwise you run the very real risk of deceiving people into thinking that A is affiliated with B.
What's so surprising about the fact that someone would find that immoral due to the potential to mislead? I would find it immoral, as well.