I wouldn't go as far to say indies shouldn't make superheroes, if that's what they want to do they should put the effort to make it stand out more than others. But at the same time don't believe that superheroes is the only genre to make. That's the problem with Marvel and DC, they're trapped in this niche therefore don't wanna try other genres. Constantly reconning, rebooting, re-everything on the same products over and over. Marvel and DC may have the high ground in the "superhero environment/population" department but it's a matter of time before that environment/population falls apart. It's a matter of time before readers want something different and new it's just I doubt Marvel and DC will do so.
That's why the indie comics market exist, we have ideas, we have stories and we want to make them. Some people keep claiming that artists should make a product based on "what's popular". Which honestly that just limits the creativity of the artist; to me it just translates to "oh just make a carbon copy of something that already exist." I mean when Twilight got so popular I bet you can't go to any bookstore without running into Twilight knock-offs. So again if indies want to make superhero comics be different about it don't limit your creativity by making a Superman-equivalent or a Batman-equivalent or a Spider-Man-equivalent.
So although I agree with this article but I think that's harsh to tell indies "never use superheroes" and "they'll never own their success".