They know damn well -- and so do the rest of us -- that if they falsified their resumes and were found out, they'd be fired on the spot.
Yahoo's stonewalling the issue right now, as Kara Swisher reports on All Things Digital, but at some point it and Thompson will have to explain how the CS credential was added to his biography. It didn't happen by itself, and the anticipated excuse -- some unknown underling was responsible -- isn't going to wash because underlings normally aren't in the business of inventing degrees for executives. Seriously, who's going to say, "I think I'll throw in a CS degree here. Scott's a smart guy, so he must have one!"
Indeed, it'd be just the opposite. When you're responsible for getting your boss's biography right for a website and SEC filing, you are going to be extra careful about accuracy.
Yahoo will not be able to successfully make this go away with either silence or a "Scott doesn't know how it got there, so let's move on" defense. And unless they produce a specific person at eBay or Yahoo to step up and take the fall, that's all they'll have to work with.
And it won't be enough.