The ancestor comment was complaining about the poor experience of installing a downloaded Linux desktop application.
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.
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.
I don't think comparing "state of packages got off of apt" to "state of downloaded installers for Mac/Windows" is an apples to apples comparison.