Oh hey that's an edit that I made. I guess I don't have enough street cred in the Dominion community to get recognized yet D:
I fully expected someone to ask at some point, why Citadel is the oddball that needs errata to work with the new rules about playing cards. So I'm surprised it took 4 days for that discussion to happen on the forums. So here's my attempt at an explanation; we'll see if it works for others.
Royal Carriage (and also Scepter) technically aren't Throne Rooms. Sure, they play cards, and they can stay out in play when they play Durations. Sounds a lot like Throne Room. But Necromancer and Overlord can do those things too, and I wouldn't call those Throne Rooms. And Citadel's wording is closer to Royal Carriage instead of Throne Room - it plays the card a second time afterwards, instead of saying "play a card twice."
(If you think this is weird, well, this is the game where "+1 Card" ≠ "draw 1 card" ≠ "put a card into your hand. Thanks, -1 Card token and Way of the Chameleon!)
Now why does this matter? Here's the relevant rules change:
"An effect that tries to play a card for the first time can only do so when the card is where the effect expects it to be. If it can play it the first time, subsequent plays will also work."
Royal Carriage and Scepter already require a card to stay in play in order to get replayed. This is why those 2 cards aren't affected by the errata. The errata is meant to deal with playing cards that have been lost, and RC/Scepter are already unable to replay lost cards.
In contrast, Citadel and Throne Room are able to replay lost cards (like Horse), and the rules change is keeping that alive. However, Citadel isn't worded like Throne Room! It's worded like Royal Carriage. But Royal Carriage isn't supposed to work with one-shots, so what do we do with Citadel?
So with the rules change, Citadel is now in a weird area. And the errata is making Citadel function like Throne Room instead of Royal Carriage.
You know, this situation reminds me Envoy. It used to say "draw the rest," which isn't rules-accurate. It just took the -1 Card token for it to matter.
Also Ingix is the person who wrote those new rules; if my explanation isn't sufficient, I'm sure they'll join the conversation. And feel free to rewrite my edit on the wiki; I wrote that without thinking too hard, and surely it can be more clearer.