Do we need "on" in this sentence?

In Steve Jobs’s biography by Walter Isaacson, the author writes:

…another health issue that became increasingly problematic, one that
medical researchers didn’t focus on as rigorously as they did cancer
or pain.

I feel like you need an additional on after they did:

focus on as rigorously as they did on cancer.

It wouldn’t be necessary if there were no they did but maybe I am missing something?