Can you share the source of where he said that? I have never heard this claim before.
The only member of the spade pirates I can find at Marineford is Skull, who appears in a few panels. And he isn't wearing a cowboy hat.
https://static.wikia.nocookie.net/onepiece/images/a/a1/Kingdew_and_Skull_Shocked_With_Whitebeard%27s_Deci sion.png/revision/latest?cb=20200915162616
I guess the best way to figure out if Masked Deuce is in the manga or not is to look at that one panel where the Spade pirates see the Moby Dick for the first time as a lot of them line-up with Oda's sketches.
EDIT: Ok so I decided to go do it and I checked both the manga, anime and the official digitally coloured manga for this and there are 19 members of the Spade Pirates shown. And in the novels there are 19 confirmed members (Ace not included). So they have all appeared in the manga and a number of them do line up to Oda's sketches (some are a little harder to place admittedly). However its worth noting that the guy in the middle in both the anime and coloured version has a long green coat on and light coloured hair which matches up to Masked Deuce perfectly.
So from all this I do think he was always planned to be a crewmember and not a novel original character.