While I don't particularly agree with him on the compatibility mode, I don't see the contradiction. His argument is that getting rid of compatibility mode provides an incentive for innovation and eliminating old unsupported code from the web. The more people that use IE9 (without a compatibility mode) would put pressure on site owners to make sure their code met current standards. Supporting IE9 on XP would increase that number.
Anyone remember the hoopla when IE8 was initially going to follow strict standards mode? They had to backtrack because site owners were up in arms about changing millions of sites to render properly. IE9 uses a lot of technology and APIs that don't exist on XP, recoding all that for a 10 year old OS so that people can continue to keep running it beyond its lifespan(and holding back progress in other areas) doesn't seem to be a good thing.
