Whose Rule is that?
The problem is, I believe, an old one. Adverbs, in present-day English, mostly end in -ly, but they are also sometimes adjectives (cf. alone as in "I studied Japanese alone rather than at school") and sometimes phrases. Cf. quick ~ quickly, but fast. There was a popular ad campaign recently that encourage its readers to "think different." Many were troubled by this and admonished "think differently." It's compounded by an ancient tendency in Germanic languages that allows underived (morphologically) adjectives to be used adverbially: e.g., German schlaf gut 'sleep well'.
I'm not using these as excuses. I, too, would probably write and say "If anybody else knows differently." But I can understand why folks might get confused.
Refactoring a written or spoken sentence is always a good idea. When in doubt, edit. And editing involves more than lexical choice.
