Take a look at the license. Specifically, read the first part (before section 1) and read section 10.
The first part says that the text is copyrighted by WotC.
Section 10 says that if you use open content, you need to include the text.
Just looking at those things and extrapolating, I would assume that Fudge used the verbiage that WotC developed (presumably because WotC hired a bunch of lawyers to make sure it was worded properly). Since FATE uses open content from Fudge, section 10 says that they've got to include the WotC OGL.
OGL doesn't mean D20, even though the OGL was developed for use with D20