Not sure how it's gaming the system. If you are rewarding players for overcoming a challenge does it matter if it was 1 giant or 11-12 Hobgoblins? Why shouldn't they get the same XP reward for the same challenge? Or worse why should someone overcoming 12 hobgoblins individually get the same as fighting all 12 at once?I think it's to prevent the PCs from gaming the system.
If you gave increased XP via these guidelines, then the players are incentivized to find hordes to crush as opposed to seeking out threats of an appropriate CR. It's metagaming, sure. But I think that's what they were trying to circumvent.