Ah yes. This makes sense to me now because you have to do 2 things to play a card:
1) Meet Requirements
2) Pay Costs
Since
NFFatRoD is a Skirmish phase event, you cannot meet the requirements to play it during the Regroup phase because of the rule you mentioned.
It's not a dealbreaker for the deck I'm trying to build, thank goodness. It'll just impact certain decisions.
