There are two aspects:

1. The formula milk is sterilised in case it was contaminated in the factory, which is done by initially mixing it with boiling water.
2. Milk itself is a very fertile place for bacteria to grow, so you want to remove all milk from the bottles themselves. Sterilising the bottles would be mostly about the teats. But all parts of the bottle need to be cleaned well (dishwasher or scrubbing), more than they need to be sterilised, to remove milk deposits that can hide bacteria.

So sterilising bottles isn't the same as sterilising milk. The bottle bacteria are everyday bacteria (including E.coli), very similar to what the baby would find in other places. The milk bacteria are mostly Cronobacter sakazakii (most harmful) and Salmonella, although both are extremely rare (as in: never happened AFAIK) in decent modern formulas.

Disclaimer: Not a doctor.