Is this speculation or fact?
This is based on my reading of the situation, so it's still speculation. Let's analyze it a bit.
What if MS knew about that but AMD did not? This would mean that MS withheld the information to affect the AMD CPU performance on purpose, which I believe is what you claim. We can neither prove nor disprove that - just speculate - unless we are provided more first-party information. But even if we assume this to be true, it is still partly AMD's fault for not noticing something that significantly affects the performance of their CPUs for multiple years when using the OS that is the most pervasive in the client segment. And my take is that I find it unlikely MS went so far to gimp the performance of one of its biggest partners on purpose.
If both knew about that it would mean it took so long for AMD to convince MS to fix their stuff. In that case, AMD said nothing about that to the public. It would mean they value the relations with MS higher than the performance of their product compared to the competition. Given how they work around some Windows shortcomings (the special scheduler on X3D and Zen 5) without MS incorporating that in the base system, it sounds strange they would not at least attempt doing something like that in the meantime.
This leaves the last option (not counting the nonsensical one "AMD knew about it but MS did not") that both did not notice. MS might have indeed not cared enough, but AMD should have - as mentioned, we are talking about the OS used by the vast majority of their client market.
Given that this has been fixed already before Zen 5 launch (STX notebooks all shipped with the new OS, apparently), this has probably already been released in the insider Windows builds some time ago. There was not much fanfare about fixing something that affects AMD CPU performance so much from either side. Maybe it was too embarrassing? Maybe the effect was deemed to not be significant enough? Maybe AMD marketing stroke again?