Anthony Mann is often listed alongside Mervyn Leroy in the credits. (although the full "lions" share of credit always goes to Leroy.)
I tried looking for information but can't find a single thing discussing Mann's involvement with Quo Vadis.
I can't give a definite answer to your question. But, like you, I wish that people who make this claim would give some details. If Anthony Mann was involved at all it could have been as a Second Unit Director. Among other things, he may have directed the Chariot Chase sequence on location with three stuntmen (one of them doubling for Robert Taylor) while Mervyn LeRoy was working back at Cinecitta with the principal players. If so, he deserves to be applauded for a superb sequence.
shareSince my previous post I've managed to track down something definite at last, through Amazon UK - Jeanine Basinger's biography "Anthony Mann" (1979, new and expanded edition 2007):
'1951[1950] ... Assigned to "Quo Vadis" and works twenty-four nights (uncredited) filming the burning-of-Rome sequences.' ('Chronology' pXX)
So Anthony Mann was a Second Unit Director on "Quo Vadis", and he directed the Burning of Rome sequence - or at least that section of the sequence that was shot at night on the Cinecitta backlot. (According to Peter Ustinov's autobiography "Dear Me"(1977) Mervyn LeRoy directed the Funeral Pyre section of this sequence on the soundstage.)
The Index of Ms Basinger's biography has "Quo Vadis" referenced for p11 - in Chapter1 'The Career of Anthony Mann' - but this page is not included in the Amazon preview. To find out more we'll have to buy or borrow the book.