Well now, I don't know if I'd call Hernandez unproven exactly. While the NWA is no longer nearly what it once was, Hernandez does have several reigns with the NWA Texas Heavyweight Championship, he's been NWA National Heavyweight Champion and had a few reigns as NWA World Tag Team Champion with Homicide. However, Kowalski had truly and unquestionably proven himself. By the time he was Hernandez's age, Kowalski had had major feuds with the likes of Orville Brown, Buddy Rogers, Fritz Von Erich and Pat O'Connor.
As mentioned, being Kowalski was the first man in North America to pin Andre the Giant and that's nothing to sneeze at. He beat Andre in his prime. Titles can come and go but there'll always be a "first" time that someone does something and pinning, arguably, the most physically dominant big man in wrestling history outshines anything Hernandez has accomplished thus far.