I'd expect that his thinking on reading the PR is that this code is OK, but could be improved by doing such-and-such, I'll do that later. i.e., he doesn't see

I'd expect that his thinking on reading the PR is that this code is OK, but could be improved by doing such-and-such, I'll do that later. i.e., he doesn't see it as rewriting but just as incremental improvement (and as such would be out-of-place as a PR comment). Ask him if he could put those ideas into PRs, as a learning opportunity for you.