They're not considering it not to be a vulnerability. They're simply saying it's outside the scope of their bug bounty program.
Man in the middle attacks may be "out of scope" for AMD, but they're still "in scope" for actual attackers.
Ignoring them is indefensibly incompetent. A policy of ignoring them is a policy of being indefensibly incompetent.
Of course, a company can do it (they just did!), but it shows that they don't care about security at all.
Especially if the answer is "sorry this is out of scope" rather than "while this is out of scope for our bug bounty so we can't pay you, this looks serious and we'll make sure to get a patch out ASAP".
Your characterization of this bug as one "that completely pwn your machine just by connecting it to an untrusted network" is also hyperbolic to the extreme.
Though, by publishing this blog and getting on the HN front page, it really skews this datapoint, so we can never know if it's a valid editorialization.
Edit: Ah, someone else in this thread called out the "wont fix" vs "out of scope" after I clicked on reply: https://news.ycombinator.com/item?id=46910233. Sorry.
The blog post title is "AMD won't fix", but the actual response that is quoted in the post doesn't actually say that! It doesn't say anything about will or won't fix, it just says "out of scope", and it's pretty reasonable to interpret this as "out of scope for receiving a bug bounty".
It's pretty careless wording on the part of whoever wrote the response and just invites this kind of PR disaster, but on the substance of the vulnerability it doesn't suggest a problem.
Op and others are saying DNS poisoning is a popular way of achieving that goal.
I'm skeptical to say the least. Industry standard has been to ignore MitM or certificates/signatures, not everything.