Thanks for checking, maltagenealogy. I thought maybe I had missed something.
The article, from the Daily Mail, says he supposedly was the English branch of the family, which is in DPB Online. There are only 9 living male ROTHSCHILDs (some only in remainder to the baronetcy) in this branch of the family, none of them with even second or third names named "Will" or "William."
Maybe it was someone who said he was part of the family, but actually wasn't?
Brooke