The Signal developers have the freedom to distribute their application in any way they see fit. If they don't want to support your machine then that's their choice. You're free to set up a fork if you want to see change in this regard, as long as you don't violate their trademark and terms of service.
I don't use gnome-calculator on Windows because there's no official Windows distribution available for that platform. That doesn't make the Gnome devs lazy, that's just my preferences conflicting with theirs.
You're not entitled to support for your preferred distribution unless you sign some kind of (probably expensive) support contract and other people not working for you for free doesn't make them lazy or stupid.
I'm well aware of Signal's history with non-official distribution and alternative clients. How they distribute their official binaries is still up to them, though. They clearly don't care enough about AppImages (and other means of distribution) to put any effort into them.
I don't use gnome-calculator on Windows because there's no official Windows distribution available for that platform. That doesn't make the Gnome devs lazy, that's just my preferences conflicting with theirs.
You're not entitled to support for your preferred distribution unless you sign some kind of (probably expensive) support contract and other people not working for you for free doesn't make them lazy or stupid.