Less. If someone has root privs on your phone they can screenshot without your notice and send those shots wherever they want, encryption or self-deleting messaging apps be damned.

Because it's largely irrelevant? If someone says "This is the best lock you can use on your front door" and someone else replies "not if you leave the key in the lock" do you think that's a important thing to mention? No lock is safe if you leave the key in the lock. Maybe it's important to remind people that you shouldn't leave your key in the lock, but that statement says nothing about the security of the lock.

Similarly maybe it's important to remind people that their phones might already be rooted, but that statement says nothing about whether Signal is good or bad from a security point of view.