I believe the guy is saving best for last. He's already released the so-so cards to flesh out other expansions and he's said that he's withheld cards for opportunity's sake and then there has a been a great deal of testbedding to identify where some real game changing stuff can be inserted.
It's just not like this at all. Every expansion in its day focused on being as good as possible. Stealing cards from later sets was fair game. Generally if the card was a great fit for the later set then it wouldn't be stolen, but I have even moved some of those, with Apprentice being a stand-out example (it was from Hinterlands). Later expansions in turn got cards from earlier sets when they were a much better fit, or, more often, when they needed fixing up.
Here's a chart of card origins. Some of them changed a lot, and I'm not including anything about the Seaside/Hinterlands split. I may have missed something.
main set: Chancellor and Feast from Intrigue; Bureaucrat from Prosperity; Council Room and Spy from Dark Ages; Library, Gardens, and Festival from Alchemy; Adventurer from dissolved small expansion.
Intrigue: Masquerade and Wishing Well from Prosperity; Tribute and Swindler from Dark Ages; Bridge from Alchemy; Shanty Town and Duke from that dissolved set. Trading Post from the main set.
Seaside: Smugglers from Dark Ages; Pearl Diver and Sea Hag from Alchemy; Treasure Map from dissolved set.
Alchemy: Apprentice from Hinterlands; Herbalist and Philosopher's Stone from Prosperity; Vineyard from the main set.
Prosperity: Trade Route from Dark Ages; Bank from Alchemy; Peddler and Grand Market from a 2nd dissolved small set; Quarry from Seaside.
Cornucopia: Horn of Plenty from Intrigue; Horse Traders from Seaside; Menagerie and Followers from Prosperity; Tournament and Hunting Party from Dark Ages; Jester and Diadem from Alchemy; Farming Village from Hinterlands.
Hinterlands: Noble Brigand, Cartographer, the bottom half of Trader, the top half of Inn, and the bottom half of Fool's Gold, from Dark Ages; Embassy from the 1st dissolved set. Edit: Jack of All Trades from Cornucopia.
The main thing you see from the cards that left Dark Ages is, that Dark Ages originally had a "non-attack player interaction" theme, but I had to give every set some of that. As previously reported, nothing to see here.