You know that really isn't true, don't you?
In Clemente's first two post seasons, the 60 World Series and the 70 NL Championship series, he hit a combined 12-43, .279, with 0 doubles, 0 triples, 0 home runs and 0 walks. So a .279/.279/.279 slash line. That's really bad. In the 71 NCLS he did hit .333, but again that was all singles, no extra base hits at all, and one walk. Then he was outstanding in the 71 Series and was pretty good in the 72 NLCS as well.
On the other hand, in the 70 NLCS Stargell's slash line was .500/.538/.583. He followed that up by being absolutely awful in the 71 NLCS and was about as good in the WS that year as Clemente was in 60. And he was awful again in the 72 NLCS. Then in the 74 NLCS he slashed .400/.438/.800. Which is, of course, really good. And then in the 75 NLCS he was pretty bad again. And then in the 79 NLCS he slashed .455/.571/1.182 (I'd have to look, but that might be the best playoff series any Pirates' batter has ever had), and followed that up with .400/.375/.833 in the WS.
For their careers, Clemente's post season slash line was .318/.354/.449. Stargell's was .278/.359/.511. Clemente had seven total extra base hits in his 26 post season games. Stargell had seven total extra base hits in the 79 World Series alone.