I've never heard them say that. The plan was always to split book three over two seasons and I seem to remember hearing them saying books four and five (which they're presenting in internal chronological order rather than split up by character like they were in the books) might even take three. This series is a pretty big cash cow for HBO. I don't see why they'd set up an arbitrary restriction on the length of the show when it's pretty obvious that doing so would probably mean cutting away more material than would be reasonable for a pragmatic adaptation. It'd be like adapting The Eye of the World into a two-hour film (which has actually been proposed by the idiots who controlled Jordan's film rights) - you'd end up cutting away so much material that it'd no longer be recognisably The Eye of the World. People claim that not much happens in books four and five, but there's no way you could condense that material into ten hours of screen time without making it no longer recognisably A Feast for Crows and A Dance with Dragons, especially since A Dance with Dragons is even longer than A Storm of Swords. I imagine the last two books are going to be even denser, plot-wise, considering how much of books four and five is introducing new characters.