That's not entirely true.
I don't suppose you never heard about a psycho-acoustic effect called the masking effect. It can affect the sum of several sources off course but also a single source.
Audio 101 really.
A source can appear to sound dull because of excess of low end content (which by nature is prone to mask upper frequency content).
However an EQ theoretically can't create what isn't there in the first place be it by boosting or cutting.(actually it can with harmonic distortion but that's beside the point)
Yes. And as the term psycho acoustic suggests, it's about the fallacy of perception.
I'm not disagreeing with you, but it's a matter of reference points. If you're listening an isolated single source, then yes, it can work. But if you reference the sound in context of a mix, you're fuck'd. "Gee, this guitar could use some more high end, I'll cut the mids and bass a tad, so it seems brighter", doesn't really work when you have a mix running and you've noticed that the guitar lacks highs.
Same for the whole mix.. "My mix seems to lack brightness and highs, I'll cut away some of the low end and some low-middle mud". Yes, it can work. Then you listen to reference mix and notice that it destroys your mix because it's brighter AND it has the low end / low mid information you eradicated in order to create an illusion of brightness.
So my point is, reference points make all the difference. If it lacks highs, it still lacks highs after you've taken out the 'mud'. You just don't notice that because your brain fucked up because it lost the original reference point.
E: Plus you are describeing a different scenario, as I stated earlier. You example doesn't need more highs. It needs less low end. The trick is to know these scenarios from another.