Did you read the article? If you know of Numba works, you know it can't just pick up different functions from sklearn and scipy and do interprocedural optimization (IPO). For Numba to do that, it'd need all functions involved to be written in Numba @jit style, whereas Weld would work directly on the pre-existing functions.

Rust is just a IPO driver of sorts here.

I'm not critizing Numba btw, I use it regularly, but your comment seems a little off here, considering that Weld has different goal in mind.