Because it's pretty clear it's either a shill account or they totally missed the point.

From looking through his website (which I found on the watermark on the photos he posted to another subreddit), this is obviously not a professional product advertisement.

None of the photos in that gallery have a watermark on them listing the product website, yet they seem to have a link. Let's assume they looked it up.

Given that he has a section of his website dedicated to photography, I think he most likely took those photos himself. These certainly aren't professional product photos.

Not true, some of the very same photos are listed on the professional website this guy just linked to, but either way taking the photos himself is moot, no one is claiming otherwise, but it isn't really an argument that it fits well in DIY either.

He certainly has access to some fancy equipment, but again, from his website, I'd probably guess he is some sort of design student and thus is able to use 3d printers, laser cutters and CNC milling machines.

This here is intentionally misleading. He stated right above that he knows the guy works professionally designing cases, even provides his company website, so why would he speculate that he may be a student who has access to those machines?

Lastly, this isn't even an advertisement. The closest the post gets to advertising is him stating that "I actually designed this case myself, and am co-owner of the company that sells them"

That is an exact description of how companies try to market on Reddit, by using ads disguised as original content that links back to their products or websites. You can argue whether or not it's a bad thing, but there's no doubt that it's advertisement, especially given how broadly they've distributed it.

So there's that. Happy now?