So if the only thing "over" (which is a comparative word) can target is other cards, and if it's not a bad thing if a card is stronger than other cards, well then "overpowered" is obviously not an applicable concept. (I've been saying this for years now.)
As Dominion does feature a fixed and not a rotating card pool like other deckbuilders, gama balance is indeed not that sensitive to varying card strength. As you neatly pointed out via Donate, Events are even less sensitive to them and e.g. Summon being virtually strictly stronger than Demand is not a huge issue.
All this does not mean that card strength does not exist and that you cannot, and when analyzing fan designs should not, deeply care about it. It is the first thing you do when you check a design, is it too strong or too weak. It is also far easier to do than analyze how interesting and fun a card is.
This is also the case with official cards. I will never understand how Cultist and Rebuild got through any playtesting in Dark Ages, they are far too strong and centralizing which is why they suck.
Back to the card, you yourself pointed out sitatuons in which Alcove could be far too crazy. And even though this does perhaps not imply that you run more than one, perhaps two, copies of Alcove, even though Alcove as a gainer of one-shot stop cards can hardly make the game wild, this could nonetheless be a serious issue which makes the card overpowered.