It's not contradictory, just poorly worded. These decks are based around a core of cards supported by other cards from external sets. The only way a deck like Goblin Bidding was as stable as it turned out to be is because 7th had City of Brass in it. An external card supporting an otherwise single block deck.

The suggestion is that even if a deck needs a set of highly specific cards to work (read Energy) it should still require some other support network, like lands or spells found in other blocks.