Repeatedly.
You are right that's one or the other. Probably both are contributing.
https://www.cnbc.com/2025/04/29/satya-nadella-says-as-much-a...
Note that he said written by, not assisted; and also that there's been some major QA issues with win11 the last year, enough for Microsoft to have made a public apology/commitment to improve...
So, no definitive evidence, but the rest of the breadcrumbs provide enough to say that yes it does appear to have suffered from it.
Given the Debian policy seems more geared towards aided/assisted an existing dev than replacing them and shipping because the code works on the golden path on a devs monster pc ... Well I'd still keep an eye on it but trust OSS OS engineers to have some competence at knowing if this works and adjusting the policy later if it proved to be wrong.
Now the argument that genAI is helping them find and fix bugs faster has a lot of merit because it can be pretty darn good at that. But for lack of a better phrase, 'The Windows 11 Experience' feels, as other users pointed out too, either vibe coded or like the dev teams went insane. I live and die by Debian in my servers and I really hope the same kind of thing doesn't happen to them.
The rumor is the opposite, that Microsoft has actually used AI to discover and fix common security issues at scale.
the entire industry is experiencing this as model capacity increases. How does "all projects get critical security fixes now" relate to "crap apps and loopy services by literally millions of lines of slop code" ?
Your appsec engineers will be the ones bug hunting with Mythos or whatever. Your regular engineers will be the ones implementing features. If the feature is a bad design and functions badly as a result, that is entirely orthogonal to whether it contains coding errors or bugs.
Also, there are bugs on the integration side that won't be caught running Mythos against source code, because it's not a source code issue. Even using Mythos for DAST is going to be very limited compared to how actual users will move through something as complex as a 'mature' OS like Windows.