I believe it does not work with WikiTree's Enhanced Editor. Personally, I prefer not to use spell check in the bio. Too often I am typing text from 200-300 years ago and their spelling did not match ours. In addition, I don't want it notifying me every time I type in a variant spelling for someone's name. It would probably also not like the coding, references, etc., that are being included in the biography.
If you are unsure of the spelling of a particular word, you can, as Joel said, turn the Enhanced Editor off and then back on again.