Yeah, I agree. But note that this can put you in the odd situation of having to reveal a card that you may have actually lost track of (because it's been covered up in the discard pile).
How could that happen? You reveal Tunnel when you discard it; presumably, that means you are holding it and putting it in your discard pile, and as you do that you reveal it.
So, that's what a normal person would do. I play Cellar, I discard four cards, and then while they're on their way to the discard pile, I show you that two of them are Tunnels and I take my two Golds and put them in the discard pile too.
But if you follow all the rules about when things happen, here's how this works:
1. I play a Cellar and discard 4 cards. You don't see what any of them are until after they're in the discard pile, and then you only see the one on top. Say it's an Estate.
2.
Now that I've discarded them, I have the right to reveal the Tunnels I discarded. They both trigger simultaneously, so I choose the order to resolve them.
3. I resolve the first Tunnel—i.e., I reveal it, which means I have to root around among the top four cards in my discard pile to find where it is, and I pull it out and show it to you and put it back.
4. I gain a Gold. It goes on top of my discard pile.
5. I resolve the second Tunnel—so I hunt through the top
five cards in my discard pile to find it, show it to you, and put it back.
6. I gain another Gold. It goes on top of my discard pile.
7. If I'm not cheating, I don't claim to have discarded a third Tunnel, look through my discard pile again, and pull out one of the ones I've already revealed. But if I am cheating, you don't really have any way to tell.
So following the rules
exactly makes it easier to cheat, somehow.