I think they did it based on what he did to Archie Moore and Jersey Joe Walcott. Though both of them thoroughly outboxed and spanked him, in the end he caught them with big punches and took them out. Now Ali I believe was a bigger man then those aforementioned and he could take a better punch.

Also I really think Marciano's Career is inflated due to the simple fact the named fighters he fought well all over the hill. However he always brought pressure and kept swinging after he missed which is quite terrifying if you're a counter puncher going against him.