Well, there's probably someone outside of Iceland that heard of Sigur Rós before Von. I know they got their break with Agaetis Byrjun, and that's when the buzz started to spread. As for people hearing Von first, I don't think many SR-fans would say "start with Von", but if we're talking hypothetically, there are people who like to start with the first album and work their way chronologically, and there are people who don't download music, and maybe they just happened to stumble across a copy of Von at the local recordstore, picking it up by chance.
I agree that someone holding Von as their favorite is rare, but apparently it happens, and they're not "wrong" since it's all opinions, if they like it more then they probably have their reasons. I rarely listen to it, but it is quite different from the other SR-albums I would say. A bit more dense, "unpolished", but it has "Hafssól" (which was really beautiful on Inni) and the title-track is quite great as well.