Actually he said they were planned for quite some time, which really doesn't answer any definite decision period. I mean anyone can say anything AFTER they had already started putting the rocks on the back wall. You can also very easily build a structure then figure out what to do with it later...up to and beyond 15 years later...which is exactly what they're doing with Lex Luther at Magic Mountain.
I'm with Coaster Freak on this one, although we'll never really know. Not really important, they're doing it with rocks and that's really all that matters. But boy, I can already imagine the uproar if that was just a wall!