Sounds like only people buying the physical sets will have access to them in the game.
If I understand you, then no, not so. It will be possible to have online Seaside without having real-life Seaside, etc.
Actually I was thinking this would only apply to Base Cards, Dark Ages, and Guilds, since the site mentions redeeming codes for them. Plus the
extra card thing in the Base Cards set points the user to playdominion.com/base, which is where this message is. I figured this couldn't apply to the past sets since they were released before playdominion was developed and none of them had that extra card which seems to be connected somehow.
So if you don't buy the physical sets, and you don't need to in order to get these card codes, but Jay said that the card was hopefully useful for everyone, and it seems connected to the online version, what I'm now thinking is this: when playdominion.com goes live, it will have all the cards from Dominion through Dominion: Hinterlands, but a code will be needed to use Base Cards through Guilds, and this code will be available for purchase, or also included in the physical sets.
Is that anywhere close to accurate?