I've worked for some large CDNs that have video services and I have no idea how I would test 8 million actual video streams at once. I mean, I have a general idea of how I could pull that off but I have absolutely never tested anything anywhere near to that degree and I don't think I could get every single cloud I have available to provision that many containers/serverless/lambdas to even test that many at once.
There was a point in my life where I had to deal with Lady Gaga (et al) releasing new music videos on Vevo.com which was probably the biggest load I've seen on any service and we absolutely did not stress test millions of streams prior to a release. I'm pretty sure we didn't even know they were releasing until I walked in that day.
AFAIK Twitter/Musk brought all of their systems in house so they'd be severely limited on hardware scaling because they lost aws/gcloud/etc scale speed.
It's absolutely possible to do this, I'm not saying it isn't, it just seems like a pain in the ass logistical nightmare so I doubt they did it. What would you do? 8 million HLS/RTC/TSP processes spread globally to not hit ulimits/socket limits/edge cases that are doing a single video stream GET and holding the connection open?