I know it’s mentioned that MCC can’t do it because of the code, but will Infinite have it?
That may very well be something we have answered in the April update. At this point we don’t know, but I would be surprised if we didn’t see it.
The Steam page for Halo Infinite certainly hints very heavily at crossplay functionality for the game, so I would be very surprised if they didn’t have it, especially since Microsoft’s last big game release on both PC and console, which was Gears 5, had cross play for both campaign and multiplayer.