They are often quite intelligent and very productive, but aren't inclined to think a problem through to completion.
I used to have some imposter syndrome because I wasn't that hyper productive, but not after having to clean that code up.
For example, I found a supposedly queue-less parallel map / threadpool with 2 or 3 queues inside of it, which had weird deadlock bugs that would result in strange occasional timeouts. I replaced it with a parallel map with no queues built on top of a thin and standardized threading library. The deadlocks went away. The resulting code was pretty boring in readable. I never bothered to dig into why the old code deadlocked, but throwing it all away fixed all the problems.
It is a lot easier to greenfield where you can put off the edge conditions till later and focus entirely on the first 80%, it is also really easy to greenfield when you focus on just typing as fast as you can and spray sloppy code everywhere (actually sloppy code like that crazy parallel map implementation I threw away). I suspect the latter is what a lot of "10x developers" actually are.
I’ve deal with with the aftermath of with “80% developers” my entire career. In two weeks, they’d write a good enough program with no handling of edge cases or errors.
Replacing one of these with a proper application can take a year.
I’ve also dealt with a team of programmers who couldn’t come close to my productivity solo. It was due to constraints and communication overhead, not incompetence.
When you go from a solo dev to a team of 19, that sounds more like a change in strategy by management than anything else.