I presume Mr z is referring to the hypothetical that had Walsh been of "normal" fast-bowler stature in terms of career length, he'd have taken maybe 300-odd wickets at 25-ish.
The point I'd make, though, is that Walsh had this over all other great seam bowlers except Glenn McGrath. He wasn't one of the very best, though still firmly in the upper echleons of the fast bowler's pantheon, but he did do something that, to date, only one other has done, in playing for 13-15 years and well in excess of 100 Tests.
Walsh had something over almost everyone else, even though he was not as excellent as those at the very top of the tree (and let's face it, there's precious little to divide almost all of the names I gave in my first post of this thread).