Yes, but as I said they can merge in changes. Apparently more than I thought, but still, they can.

Opera has pinch-zoom text-reflow in a chromium backend, and that seems to be substantial, and yet it is (on purpose) kept out of mainline chrome. So they do loads of tracking/merging too.

The scope of work to do a few small features on top of chrome wouldn't be a biggie, compared to the entire project.