Now admittedly this is sampling bias, but I have never seen code written by a quant that could be put into production without being reworked by a software professional.
The skill sets are in some way exclusive of each other. Quants do lots of exploratory work. This involves tons of one-of experiments and throwing away things that don't work. They often frequently don't have the operational understanding of the way the systems they interact operate. Further, making stuff fast on a computer is a very, very particular skill. People that could do all of that are extremely rare (such that I've never met one).
I can personally vouch for the fact that there is a large space between "can't code" and "write high performance production quality code". Quants are no exception.
And a large space between "can't write high-performance production-quality code" and "doesn't write high-performance production-quality code".
I can write high-performance production-quality code, and occasionally I do, but maybe 95% of the code I write is hackish, primarily intended for exploration and experimentation, and written in a language fundamentally ill-suited for high performance. Engineering it all carefully and optimizing it would be a tremendous waste of time and effort.
I work in technology rather than finance, but I bet there are plenty of quants for whom much the same is true.
The skill sets are in some way exclusive of each other. Quants do lots of exploratory work. This involves tons of one-of experiments and throwing away things that don't work. They often frequently don't have the operational understanding of the way the systems they interact operate. Further, making stuff fast on a computer is a very, very particular skill. People that could do all of that are extremely rare (such that I've never met one).