I am a developer, but only a 1X developer.
But I can say without a doubt that I have met a small handful of people who are many, many times more productive than ordinary developers. Call them 10X if you want, or dismiss the idea that 10X exist, but fact is there are developers who are simply streets and miles ahead of others in terms of productivity.
Why people claim 10X developers don't exist surprises me. Maybe they haven't met any or weren't able to recognise such productivity.
One important thing to take into account when considering extremely productive developers is context:
happy personally productivity +1
unhappy personally productivity -1
deep experience with language/framework/tools/tech productivity +1
average experience with language/framework/tools/tech productivity +0
little experience with language/framework/tools/tech productivity -1
inspired by the mission productivity +1
ho-hum about the mission productivity +0
skeptical about the mission productivity -1
likes the boss/management/fellow team members productivity +1
dislikes the boss/management/fellow team members productivity -1
not impacted by organisational politics productivity +0
impacted by organisational politics productivity -1
provided best tools for development productivity +1
inadequate tools for development productivity -1
knows the codebase productivity +1
does not know the codebase productivity -1
etc etc on and on