> Limited training data, as compared to kanguages like Python or javascript.

I use my own APL to build neural networks. This is probably the correct answer, and inline with my experience as well.

I changed the semantics and definition of a bunch of functions and none of the coding LLMs out there can even approach writing semidecent APL.

I agree with you, though in the q world people tend to take it to the extreme, like packing a whole function into a single line rather than a single screen. Here's a ticker plant standard script from KX themselves; I personally find this density makes it harder to read, and when reading it I put it into my text editor and split semicolon-separated statements onto different lines: https://github.com/KxSystems/kdb-tick/blob/master/tick.q E.g. one challenge I've had was generating a magic square on a single line; for odd-size only, I wrote: ms:{{[(m;r;c);i]((.[m;(r;c);:;i],:),$[m[s:(r-1)mod n;d:(c+1) mod n:#:[m]];((r+1)mod n;c);(s;d)])}/[((x;x)#0;0;x div 2);1+!:[x*x]]0}; / but I don't think that's helping anyone

I don't think that's the biggest problem. I think it's the tokenizer: it probably does a poor job with array languages.