You should visit Orlando on any day in July or August between the hours of two and three PMYes it can rain on a bright day but deluges as is being mentioned nearly always means clouds full of rain i.e. dark clouds. Not making excuses for Tesla not turning lights on automatically but I cant remember seeing a deluge where I cant see, on a bright day?!
In MA, I believe the law is that if your wipers are on, your headlights should be. There's really no downside to that.