WINE wasn't originally licensed under the GPL. Because of Cider's predecessor making a commercial fork of WINE, WINE relicensed to GPL to prevent further incorporation of WINE code without releasing source. Thus, Cider is ultimately based on a really old version of WINE. A lot of it will be original code.

We're LGPL, actually, precisely so commercial software can use Wine to do their ports.

Ah, I didn't know it was LGPL. Anyway, that's true, but you have to release the source.