Requiring the MPV to know which address/port the Host is listening on introduces unnecessary coupling in most situations. If the config file doesn't specify this information, the IG should fall back to just using the remote endpoint address of whoever is sending it IG Control packets--which, by definition, is supposed to be the Host. There's generally no need to pick a particular Host address/port and 'hard code' it in the config file. (Yes, it's in a file, so it's not strictly 'hard-coded'; but changing the Host address and/or port currently requires modifying the MPV's config file. Even this wouldn't be necessary if the IG would just respond to whoever starts talking to it.)
This opens the door to DOS attacks (session 'hijacking' by a malicious interloper), so it might not be appropriate in all situations. But the vast majority of installations I've encountered are deployed on trusted internal LANS where it can be assumed that any IG Control packet received is coming from a valid Host. And the use of this fallback mechanism isn't mutually exclusive with the idea of hard-specifying the Host address in the config file; that could still be done where necessary.