if hg-git uses dulwich, and it's all pure python, how come i see it running git-index-pack amongst other git (what i assume are shell) commands?

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.

i'm pushing to github. reason i ask is that it segfaults -- see https://github.com/schacon/hg-git/issues/216

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.