I don't get your point. You are doing an appeal to authority, yet you seem to be literally arguing with history and entire industries as to what can make a very scalable system. Or, is your argument that Twitter would be more reliable if it were done with QNX? What about Amazon? Google? Any behemoth out there?
If that is your argument, then it seems a moot subject. We'll never know. If Twitter had originated on a microkernel based operating system, would it have gotten the traction it did? Beats me. Not even sure how you could approach such a question.
Now, I fully agree with the view that historically a major way to get these was to go with a solution like QNX. My point is merely that the reliability of other solutions is not as far behind as they used to be. To the point that limiting yourself to only run on one of these systems seems overly restrictive. (And again, that is in today's world. Not the one of 20+ years ago. Probably not even the world of 5+ years ago.)
This was not a theoretical exercise, it built a very deep and wide defensible moat, every Monday was a day pushing a record number of messages through the system since its inception, in other words, growth was continuous and downtime unbelievably (by todays standards) low. Which had a lot to do with the intrinsics of the OS.
And you have my word that without QnX as the cornerstone of that development it would have simply never happened. You can believe me, you don't have to but you're asking me for my reasons and then you proceed to dismiss them on the basis of your theories.
Practice > Theory any day.