[Bagger Lance]

I think he might be referring to Bobby Clampet's book 'The Impact Zone' rather than the Yellow Book.
Alex

In those pages, Bobby discusses how "game improvement" club designs can hinder players from developing a more dynamic swing.

Baggers Summary -

The premise is that a lower/rearward clubhead center of gravity plus clubface offset help produce higher shots but due to the nature of their design, conditions players from learning how to hit down enough. As a result someone conditioned to using game improvement clubs have difficulty when moving to blades. Hence the general consensus that blades are reserved for better players.

In reality, blades (or blade-like designs) are the best game improvement club because they don't reinforce compensations.