I'll make two arguments, and you can pick one.
You know the old saying. Batman always wins, if Batman has time to prepare.
If this were the other way around, and the players were the ones doing the buffing, would you penalize them for it? Would you LOWER the amount of XP if they prepared for an encounter to make it easier? If so, yep, go ahead and give them MORE XP for this one. If not, well...
I argue that: The encounter difficulty is based on the advantages available to them, vs. the level. That the enemies were smart enough to optimize should not affect XP.
And really, if the party has encountered these folks before, shouldn't THEY be optimizing for what they have seen in the past?
It's different if it's a total surprise.
Really your call, just my opinion. The other argument is that since this is tailored specifically to screw them, it should give them more XP, because it's literally more difficult to deal with. Maybe you should see how the fight goes, and read the mood in the room. If they sacrifice a lot to get through it, up the XP. If it's much easier than you thought, stick to the strict definition.