Any case that makes you say, "a character shouldn't be able to do that," should be carefully examined when that ability is identified as supernatural.
Can a monk run faster than an accelerating motorcycle? Naturally not, right? Supernaturally, however.... who's to say?
Shadowdancers that are attempting to hide in plain sight are breaking normal, natural laws. Thus their ability is supernatural.
The ability does, however, require some shadow, which the scenario above does not include. I think this is simply a case where following the rule as written will work perfectly fine. The shadowdancer should not be able to hide in plain sight in the scenario because there is no shadow, as opposed to him not being able to hide in plain sight because there is no cover... these are two different things.