Your logic makes sense re your non-Fortress examples. And maybe also makes some sense re Fortress...I need to think about that a little bit.
But in the example Grindcore gave, he was not gaining a Fortress *back.* I would agree that if you trash a Fortress in your hand (Chapel, Upgrade, etc), then when it reappears, I can see how that game does not see that as a gain. (I also see the merits of seeing it AS a gain, so either decision is fine with me.)
But with Lurker, you get a Fortress put into your hand that you never had before, so it seems like tortured reading/reasoning to not see that as a true gain.
(I'll note that I see the fact that this game has a ton of edge cases as a feature and not a bug...I love the complexity that comes up with unusual card combinations.)