One possibility is that as the number of slackers grows, the amount of damage that they can do increases at a greater-than-arithmetic rate, particularly if they tend to cluster together.
For example, if 5% of a company overall is composed of slackers, but 25% of their customer support department are slackers, they may do outsized damage.
" Fundamentally, everyone wants to do a good job (the statistical outliers who do not follow this are not worth focusing policy on)"
You have just said you should focus a policy on getting rid of them. I agree with this, but it goes against what the article just said to do.
I posit this is because he does not have a large enough company to have enough slackers that he needs to deal with them systematically, and instead can simply deal with them on a one-off basis.
For example, if 5% of a company overall is composed of slackers, but 25% of their customer support department are slackers, they may do outsized damage.