So that's a quality answer...except for the Tunnel waffling bit. So if a reaction can be triggered by a card that is being placed into your discard pile, it is not clear to me why this disallows something going into play from your Tavern mat being a reaction. That said...it being a Reserve might basically cover the case, anyway. And, to date, there are no cards that allow you to react to reactions specifically...I think?...so maybe it's just a metaphysical question, and metaphysics doesn't really pay.
"Reaction" isn't "this has an ability that functions other than when played." Obv. Mint, Catacombs, etc. have such abilities and are not Reactions.
Instead the idea to "Reaction" is to use color to help remind you that a card is usable at an unusual time. When-gain and when-trashed already involve the card, so we don't need to draw attention to the card. But Moat is just a bystander to the attack; we can help you out by calling attention to it.
Tunnel being discarded involves Tunnel, but it seemed good to call attention to it anyway.
Reserve cards as a group function at unusual times and are a special color. They've got this covered.