You're right. It doesn't. But it's a matter of degree, not kind. Knowledge work has many times the variability of manufacturing (by design: if you remove variability from knowledge work, you're no longer producing anything new each time.)
In other words, this applies even more severely to knowledge work.
More concretely: in manufacturing you can have a process that yields 9–14 % defective or whatever. The variation is relatively small; say a CV of 10 %. In knowledge work, you'll be looking at processes that generate somewhere between 0.1 and 100 defective ideas for every really good idea. This variation is enormous: 1000 % or so.
In other words, this applies even more severely to knowledge work.
More concretely: in manufacturing you can have a process that yields 9–14 % defective or whatever. The variation is relatively small; say a CV of 10 %. In knowledge work, you'll be looking at processes that generate somewhere between 0.1 and 100 defective ideas for every really good idea. This variation is enormous: 1000 % or so.