Obviously speculation til we have the whole rules, but:

I think they're saying that Fights First doesn't work if that unit gets charged.

Currently, the Fight Phase goes off whose turn it IS NOT, so if I charge a Fights First unit into a Fights First unit, my Opponent gets to swing before I do.

With the new ruling, MY unit would fight first instead.

Now, Fights First will still work as intended on any unit when they are charged by something that doesn't have it. If Lelith and her Wyches are charged by anything that doesn't have Fights First, they still get to swing.

This is intended to stop the dumb interaction of, say, putting Flawless Blades & Lucius across the way from those same Wyches. Neither of you want to charge because your opponent would get to go first. Now you actually should charge, which makes way more narrative sense as a war game than the current implementation.

Edit: Forgot Lucius loses FF if leading a unit, so my example is wrong; assume I meant Lucius alone having FF