As a reminder... the video where the guy showed the amp squealing had his master volume on 8 out of 10. Absolutely no serious guitar player runs a Mark series master volume on 8. It's just not that kind of amp. If there is oscillation with the Master volume at normal settings, then yeah that's not great, but do we know if that's the case?
For the record, I have a Mark IV revB long chassis and a Mark V:25, and neither one of those amps oscillate with the Gain and Treble maxed with the Master at non-extreme settings. I just tweaked around on the IV and to get self oscillation, I had to turn both drives to 10, Treble to 10, Master to 8, AND the channel volume to about 8. Even just turning the Channel volume down a bit resolved the issue.
This was also the case when I owned a Mark IIB. I cranked both drives and the Treble on that amp all the time and never got oscillation because I only ran the Master to the absolutely deafening setting of about 3-4 which was still easily loud enough to stand out in a live band.
In short, in my experience, you won't get self oscillation on these amps unless you pretty much go out of your way to dial them like an asshole.
Maybe this is a symptom of everybody having reactive loads and reampers now. Maybe it's easier to just not realize how hard people are actually pushing their amps when they crank that volume knob. Reactive load tech is invaluable and in my opinion it basically saved big amps existing as a concept in the modern world, but people should still be aware of what is actually happening in their gear when they twist the knobs.