Python's floats are usually doubles by default, so it's mostly fine.

That said, yeah, that implementation isn't ideal. At a minimum, Kahan summation is usually free on large vectors (you're bottlenecked on memory bandwidth anyway), give or take the fact that you need to disable floating point re-ordering to keep the compiler from screwing it up and therefore have to order the operations correctly to make it efficient (see some other top-level comments about data dependencies as an example).