Yep, several projects I know are leaning into AppImage to get around dealing with installers. The biggest problem with AppImage is that it _is_ just a binary, so things like desktop shortcuts don't come with it. AppImageLauncher does solve this issue (and create a "canonical place" to place your programs, like /Applications/ on mac).
I do agree it's fragmented and frustrating where we've ended up for installers. My main critique is more that the state of linux packaging _isn't_ what thunderbird is doing here.
Yes, the reasons for why it's more complicated on Linux are varied, the fragmentation of the desktop Linux ecosystem might be the other main factor.
However I think comparing the status quo between each OS is fair.
Appimage seems cool, I don't have experience with it, so I can't speak to how well it works.