If the author wanted to write his own, he could very easily write an nd-array-alike wrapping a really long double[] in Rust, right?

From the author: You can work around it by using a Vec (arbitrary sized sequence/list) but then your matrix is allocated on the heap not the stack, meaning slower operations. Plus that means you cannot use Rust wonderful type system to check that you multiply matrices with compatible dimensions, say a 2×2 matrix with a 2×1 matrix, without jumping through hoops.

So he thought of that, then decided not to go that way, even though that's how it's done in C/Fortran underneath every single linear algebra library in the world, and his conclusion is that rust sucks?

I'm finding it a little hard to believe that there are real-world linear algebra problems where allocation is the bottleneck and doing it all on the stack is the answer.