Not true. The Romance genre has specific requirements. So it could have romance in it, but not be in the Romance genre.
Again, I have NEVER ONCE walked into a bookstore where 70% of the shelves contained ONLY Romance novels. That, plus the fact that the entire SW article is based around "how to market your book" leads me to assume that they're ONLY counting "classifications", not specific genres. They're advising authors to label ANYTHING they write as "Romances" in order to boost sales. And since they DON'T break down the numbers, there's nothing suggesting their numbers AREN'T bogus.
Again, SW allows two separate genre categories, which are EXTREMELY broad. They also allow you to add SEO codes, allowing eight of these. Since all of those allow the use of "Romance" as a category, every book has 10 separate chances to classify their books as "Romance".
We're not talking about the books that "Romance Monthly" considers to be Romance books, we're talking about all the books that SW considers "Romance" for marketing purposes, not for tax purposes. (I've read the SW annual reports for the past 5 or 6 years, so I'm familiar with their emphasis and how they compose their numbers.) Again, for marketing purposes, if you list a book about alien tentacle monster's invading Earth, but you add a "Romance" SEO code, SW counts it as a ROMANCE book (as best as i can determine, since they NEVER list how they define the category). The 70% number is simply an unrealistic figure otherwise.