He's talking about books written on eastern philosophy, not books of eastern philosophy. Granted, I doubt he's read every western book on eastern philosophy but it doesn't seem an unreasonable statement to suggest that many of them likely do not reach the same level critical analysis that Watts' output did.
I'm curious of where in Confucius or Mencius critical thinking is discarded?
Though, ironically, Alan Watts actually has a number criticisms of critical thinking as well (in both article and recorded lectures). Zen (from it's daoist roots) isn't discarding critical thinking, it is pointing out its limitations.