The author somehow seems to implicitly assumes that being able to read as early as possible is good, period. Younger == better. He has a huge "why" chapter, where he debunks and defends all kinds of criticism he's gotten, but I haven't been able to at all find the section where he simply describes why it's a good idea. Not why it's not not a good idea, that's not good enough. Why would I teach my children to read already?
There might be good reasons, but this author is so consumed with the "it's possible! it's not harmful!" part of things that what should have been the central argument is nearly entirely missing.
I mean, seriously, it's possible and it's not harmful? I can think of a lot of things that are possible and not harmful but still wouldn't do with my kids.