Worth noting that I assume the point of playtesting is not to find which cards are strong and which are weak, it's to figure out which cards are fun. It's okay for a card to be ridiculous (King's Court, Goons, Chapel) if they lead to interesting games and you still have some neat things to do in the actual game.
In this respect, I assume that a fun card in 3-player / 4-player is usaully going to be fun in 2-player.
Of course, if a card is too strong, it can stop being fun, and if a card is too weak, it's just taking up space, so you do have to care about power level somewhat...but I don't think it's the primary thing you're looking for and it seems like people are thinking about it from a standpoint of game balance / making card power levels flatter, rather than from the stuff that gets people to play your game in the first place.
I suspect 2-player Dominion is pretty robust in terms of balance - if a card is nutty in multiples, and both players go for it, it'll still break out even. So not too surprised sets can turn out well even without a lot of 2-player testing.