When someone says that someone else is smart because they got a 100 on a test they are taking a multidimensional attribute and projecting it onto a single dimension.
However, acing a tough exam does show that you have high ability in some skills which can be included in "intelligence". It is perfectly reasonable to say someone is smart after they do something to prove their cognitive ability, even if they haven't proven every single possible kind of cognitive ability.
I think the miscommunication here is what people mean when they say "he is smart" or "this hotel is better than that one". I don't think people are commiting a scalar fallacy when they say these things. People are making judgements based on the many different aspects of a thing, and then using simplified speech to express their opinion.
If you are booking a vacation, you have to choose a hotel. When comparing hotels you look at all the different aspects (the bed, room size, the restaurant/bar, etc). You have to weigh all these options and then pick which hotel you would prefer to stay at. So when you make a choice and say 'hotel A is better than hotel B', you are not asserting that hotel goodness is measured in one dimension and that hotel A is objectively better. It's rather a benign statement that you would prefer to stay at hotel A.
It's not a fallacious reduction to a scalar quantity, but at some point you do have to make choices and weigh preferences.
> pervasive assumption
I think whenever "rating" anything people are aware that it's a subjective and probably useless number meant for convenience than accuracy.