Since Signal is not a commonly available method of communication with L5 (needs work, workarounds, luck), I see this as an avenue for people who are coming from those other phones and thinking of trying L5 - they have this messaging app available easily, so this probably serves them (insofar as much as Purism has bandwith to communicate in any channel). This shouldn’t be judged so fast at this point (beforehand) - the more important thing is how information flows and what are the response times in any channel.
Trying something new is a good thing. It should be encouraged (even if this wasn’t what you may have expected or dreamed of) - there might then come other new things. And who knows, maybe Signal eventually some day becomes a real alternative [which is a different discussion]