The problem is that with baseball you can bury so much in randomness.
Look at Schilling.
Regular season
ERA 3.46
WHIP 1.14
K/BB 4.38
Postseason 19 starts
ERA 2.23
WHIP .97
K/BB 4.80
He was an excellent pitcher in the regular season but in the postseason he took it to a higher level. Look at those numbers, all against playoff caliber teams, obviously.
But what I gather is that the argument is that the excellence of those numbers may be mere randomness - that he could have had a stretch like that in the regular season as well. Well, maybe he could have. But is there really proof that it was all randomness? I don't see how.