He didn’t claim anything he proved it in his book.
How? The excerpt you linked to just claims it. Are there verified documents to prove he refined the keel shape to the extent that he actually designed it (as far as that can be defined)? As far as I recall the controversy, Sloof claimed to have invented wings and that was clearly not the case as the plans of a winglet 12 had been published around 1974.
Just saying something is not proof. If that was so, the denials would also be proof. Anyone can say anything, but that is not proof.
I only met Ben personally once but he was well known as someone who was fond of trying out wild ideas. He had been a fan of Uffa Fox, who had used "upside down" keels in the 1930s and 1940s. He mentioned his Uffa story in a Ronstan-sponsored lecture at Middle Harbour YC, Sydney, that I went to as a kid and that was reproduced in Australian Sailing magazine about 1980, so this is not revisionist history.
Many designers had done "upside down" keels (ie with root shorter than tip) in the 1890s and Van de Stadt and Spencer (not to mention others) had done "upside down" keels in the '50s to '70s, so the inference in the book that they were new thinking is obviously wrong. Sloof says that he "point(ed) out" that an inverted tape/upside down keel moved ballast lower and improved stability but come on, everyone knew that, and Ben certainly would have. After all, within a mile or so of his house there were Spencer and Van de Stadt boats with keels specially designed to have narrow chord roots and long bases to keep the ballast low, as did his hero Uffa's Flying 15s. Sloof's claim that he had to "point out" this stuff is obviously over the top.
Sloof says that the opportunity to test "upside down" keels had not come up, but he also says he didn't know much about 12s or the AC so one may well wonder how wide his yacht design knowledge was. "Upside down" keels had been around for 90 years so they were not unknown, as he seems to imply, whereas we know for a documented fact that Ben had been a big fan of two men (Fox and Herreshoff) that used them and every designer here knew about Spencer and VdS who also used them. Given that Ben had used wings on foils (I have 1963 vintage published drawings of them on my bookshelf); that he was aware of "upside down" keels; and that the two other Australian designers had published a drawing of a 12 with winglets a few years before, it's easy to see that he could have put the ingredients together and conceived the winged keel. If it was so hard to conceive, why had Payne and Hood done it years before?
Parallel invention is also very well known, as is taking an idea and effectively making it your own by modifying it significantly and putting it into practise.