Notice it doesn't say that if 51% of the computing power decides not to upgrade... never mind.
It says, we've decided. Come along or not.
It also says "likely"

If 51% or more of the computing power - would probably only take 3 or four pools - had decided not to upgrade, it wouldn't have happened. But it was in their best interest to do so because it was a bugfix, not a philosophical split.
Also, you forgot that 51% (or more) of the computing power had
already decided:
http://bitcoin.org/chainfork.htmlLarge mining pools running version 0.8.0 were asked to switch back to version 0.7, to create a single block chain compatible with all bitcoin software.
Those same mining pools were then asked to do the upgrade on the 15th.