I definitely favour an open book attitude when it comes to opponent's knowledge of my special rules. I always offer to explain and will gladly answer almost any question. I like the dice and my strategy to decide the outcome.
If he asked "Can I close it?" I
definitely would have told him that yes indeed, if he surrounded it then all my non SKimmer Vehicle mounted squads and Jetbikes would be blocked in.
I feel it's completely reasonable not to tell people when they plow away in ignorance, but as soon as they ask something like "Is there anything I can do to stop the WWP?" I would have replied yes. I would have told him that a) if he kills the model carrying it then it won't deploy and is lost for this battle. And b) if he surrounds it I can't march foot sloggers like my warp Beast pack out of it although Raiders etc could still come out.
WHen describing my army to opponents at tournies, I do always try to see if they're familiar with DE and if they're not, to run through the special rules for them. I encourage them to feel free to ask anytime. WHile a question like "How do I beat your army" is appropriately left unanswered in the heat of battle, "How do I take advantage of your special rule" is quite different, and pretty much needs a cleanswer whenever it's asked. This includes telling him, if asked, that a WWP can be surrounded, even though he's asking the question before he's moved his bikes which will be able to surround it.
Your "edge" comes from your skill in using a piece of wargear, surely? Fpr me, there is much less satisfaction from a win that was basically due to the ignorance of my opponent. Far far better, and much more fun, is the game vs the grizzled veteran who's played this (or similar) force many times before. If I get a victory
despite my opponent's full knowledge of my special rules
and exploitation of it's weaknesses it's far far more satisfying. Trust me on this, games are more fun when it's straight up competition. Sure, be sneaky and deadly, but remember it's still a game. How fun would Monopoly be if one person were able to buy hotels for whatever property they got (instead of needing sets and building up houses to get to hotels) while everyone else couldn't. They'd almost certainly win game after game, but how long would opponents continue? The point is, how much fun is it for them?
I think that guy will feel a bit miffed when he discovers that indeed he can surround a WWP. If you had any foot squads in there at all, well, it was a bit too tricky this time.