Sure it is. Both in larger program design and in affording several different ways to accomplish the subscripting into the giant nd-array Vec<f64>. Maybe Rust should also provide language support for the ndarray type, but lack of such isn't a barrier to being as-good/better than existing langauges.

If you're doing linear algebra on big matrices, putting them on the stack is most likely madness. It sounds like the author is interested in Rust for the 'expressive' type system enforcing logical invariants about his matrices, which isn't why Rust built their expressive type system.