It's not out of scope "for taking over your computer". It's out of scope for the specific goals of the bug bounty program. Bug bounties are (usually) about prioritizing internal engineering effort; they are to vulnerability remediation what market feedback is to feature/function decisions in the rest of the product.
Everyone's judging this by the standard of "how good a bug" this is. But that's not necessarily how a bug bounty should function. Important prior to frame this with: neither any individual bug bounty submission nor the sum of all valid submissions materially alters the security of a serious product, at least not on their own. The system they feed into (for instance: security engineers taking a validated bounty submission and then quickly auditing the entire tree for variants of the same bug) can move the dials. The bounty bugs themselves though are mostly a sideshow.
What's especially weird (you didn't say this, but the sentiment has popped up on all 3 threads about this story) is the idea that AMD would be trying to cover this up. Why would they care? They run a bug bounty program. They've accepted the premise that they have vulnerabilities.
(From earlier today, in add'n: https://news.ycombinator.com/item?id=48492908).