We only have musk's word that tag was used to measure visibility. Wouldn't a random sample with users of different sizes and types make more sense for that?

Even if it was, that could have the same effect. If they test every change and only keep the ones that don't harm Musk's visibility that is in effect making the algorithm consider Musk specially.

Lots of changes will be made every day at Twitter that will have to keep the Musk metric healthy. Even if the changes aren't explicitly designed to promote Musk's own tweets, having the metric visible to all employees will at minimum eliminate any change that would demote Musk's tweets. Over time, the Twitter algorithm will become biased towards promoting Musk-like content.

Would promoting Twitter Blue Subscribers content over non-subscribers be promoting specific accounts? Musk has said this was going to happen and I think we saw it in the code + it sure feels like when using the site.

Seems like that would could even be considered hardcoding - Buy Twitter Blue get higher on replies on threads.

companies have been offering paid boosts for many years though. not sure how this is different. i guess as long as anyone can buy it it's ok?