A look at the numbers
ML: "An invariant five per cent Ev game which is about the best a 100% hole carder can do if he uses reasonable amounts of play cover to hide hole carding, not hitting seventeens against a twenty and things like that, has a SCORE of $2500."
Such an edge would produce such a SCORE.
ML: "Knowing the count from traditional counting or shuffle tracking would add little because it would only apply to stiffs against stiffs."
Agreed - not enough to double the SCORE.
ML: "I can conceive of no mathematical way a 100% hole carder could ever add enough to get to $5000."
I can conceive of a mathematical way to increase the SCORE: increase the player's edge. For a $5000 SCORE a player would need an edge of about 7%. Unless Grossjean's numbers are wrong, a 100% hole carder has an edge greater than this, even with very heavy cover (much more than I would ever use) - see BC p.115.
And after all, there are more powerful methods than holecarding out there too.