I think you're possibly misunderstanding what these supercomputers are for. They just aren't designed for whatever single-threaded workload you personally have, so it's not in scope.
It is clear for me what they are for, and why I would not use it for a single-threaded task.
I was trolling a little bit, the people who downvoted my measure of speed :-) because the millions of FLOPS of a supercomputer, will help for parallel tasks but will not be "faster" for a common use case.
So fastest computer is one thing, most powerful is another.
"fastest" is accurate. You can get more computation work done in less time given an appropriate workload. No matter what adjective you use, "fastest" or "powerful", you're always within a context of an intended workload.
Your argument is a bit like saying the fastest land speed vehicle isn't really the fastest because you can't go to the grocery store with it.