Huh?

Are you pushing to a local repository? When pushing to a local repo, dulwich calls a local git binary. I don't see any calls to git-index-pack in either hg-git or dulwich in a cursory grep, which matches my memory.

Something like that is not exactly core functionality. It could be a situation where hg-git runs purely in Python for everything that it 'needs' to do, and will optionally use the native commands for other git functionality.