However, in addition to e.g. hacking the actual developer/distributor, on Windows, you have to be wary of a rogue versions on e.g. download.com that rank higher than the genuine one, and of google ads for trojaned VLCs and stuff.

It is no panacea, but it is very significantly better than the Windows world (and the Mac world outside of the app store)

Furthermore, modern snaps have limited privileges and file system views, so a backdoor is much less effective; I assume appimages and flatpaks have similar mechanisms (and if not, they should).

Situation is far from perfect, but it is about a billion time better than Windows whichever way you measure it.