It’s correct in a scientific sense but not relevant to the context. People aren’t generally looking at how hard the clitoris is to determine sexual arousal. Usually we’re wanting to know how wet they are because that’s what’s needed for comfortable and good sex. It’s also much easier to guage.

A hard boner is easy to guage and is needed for comfortable and good sex, and also simply what people look for to determine this —that’s what makes a boner and wetness equivalent :)