Chiron won the Monte in '54 in an Aurelia. And I pressume all the Fulvia (and Flavia??) victories in the '60's and 70's don't count?
I suspect it will come down to the use of the term "World Championship" which started in 1973 (as opposed to international rally victories since time immemorial), so wouldn't actually include any Fulvia victories (none in 1973 - then the Stratos' came in). Is this the same as International Rally Wins? Definition and intention: of course Ford would like to kid us with with a statement " the most successful rally manufacturer ever" when what they mean is "the most successful rally manufacturer wince the WRC started in 1973".
Pedantry matters
Grand Prix racing is going through this with all sorts of dubious claims being made about "60 years of F1".
I ought to say I don't know what the victory count is since time immemorial, so for simplicity I'll stick to Lancia are the best!