I will be sarcastic, ironic and voluntarily provocative: he is an idiot, if he was a real 10x developer, he should have chosen a complex code architecture only

I will be sarcastic, ironic and voluntarily provocative: he is an idiot, if he was a real 10x developer, he should have chosen a complex code architecture only understood by him, that way he would have been the only one able to implement and maintain features inside the code base, making all the other developers 0.1x and consequently he would have become a 10x developer himself. More seriously, this kind of situation can happen without ill intent, simply have a fast developer with a strong personality and he will become the lead for everything and the only one able to understand the code base. Also the velocity of an individual developer can be overvalued and mistaken for a 10x, it's certainly easier and faster to write spaghetti code instead of a well designed/architectured code base, but doing so can have dramatic effects down the line, or even immediately for the other developers in the team. Yet it can be a quality in critical situation (like our startup will shutdown if feature X is not implemented in 2 days). It can be hard to differentiate between a developer like I describe and a real 10x developer you describe. The applications we designed can be quite difficult to implement depending on the complexity of the domain specific logic the application deals with. Sometimes it's impossible to recognize between complexity caused by the application architecture (generally avoidable) or complexity inherited by the overall domain logic (nearly unavoidable). And lastly, it's never, ever as black and white as I describe. That being said, your colleague seems like a really good developer, able to see the big picture and steer a code base in the right direction. I hope for you that he will part of your team for a very long time.