> maybe you have some relevant context

Nothing you haven't figured out yourself. As you say, they're both private functions, and as we investigated adding a "get_target_platform" function, it turned out that nearly all users were expecting the target platform, so rather than updating everything we added one for the less-common case.

get_host_platform was not defined before, so I'd say use it if it's there and use get_platform if it's not.

I doubt we're going to add any new public/supported APIs to distutils ever again, to be honest.