Lots of moving parts on this discussion and wise / smart people have probably all weighed in without solving the problem necessarily
I'll suggest that maybe "biologically" speaking, intelligence makes smart people seek out knowledge which makes them "unhappier"; this would explain why they can't simply use their knowledge to become happier, as "ignorance is bliss" and their native intelligence naturally has them seek "knowledge which isn't bliss".
Along with this, I've experimentally noticed that what we focus on seems to influence our sense of happiness, so then you could hypothetically just try to focus on things that make you happier; with the intelligent, though, they may tend to focus on things that do not make them happier, even though they may trade happiness for a greater sense of meaning and purpose. For example, it's been said that parents are less happy but have a greater sense of purpose in life (according to some studies). Likewise, the intelligent person may try to contend with difficult and troubling intellectual issues, or may become aware of thoughts which have a greater emotional impact on them than a less intelligent person. So again, intelligent people "just happen" to likely make themselves unhappy, by virtue of being intelligent (?).
There is some fuzziness about what it means to be intelligent or happy, and yet at the same time we seem to have an understanding of what those things mean. If it is clear what makes us happy, then the smart person can just do those things. But, basically happiness can be subjective - some people derive a lot of happiness from inflicting a lot of pain on themselves (like a smart person trying to tackle progressively more painful intellectual problems). So... it's also kind of unclear if we know what makes us happy (it might vary per person), and also if happiness is even the goal in life (versus living a meaningful life, although happiness and meaning are often thought to go hand in hand).
The article notes that positive psychology cannot be used to increase happiness; I'm kind of skeptical of this view, but sympathetic to the view as well. What I did was ask AI what makes people happy and to try to do likewise, and the conclusion was kind of like going fishing, like you can go out on the water and show up, but the fish still decide if they bite or not. Happiness is like the fish - still a little outside our control, but we can possibly create some conditions where it might be likely to "bite" us.
Also there may be "multiple happinesses" - kind of again getting at the question of what it means to be happy. I've seen Christian saints describe a kind of internal sense of joy, even when externally they might be in lots of physical or emotional pains. Or by contrast there may be a sense of "inner peace" which a person could have, while their life physically or otherwise is chaotic and might be associated with "unhappiness". A clean conscience can be important and overlap with some of these things.
Anyway, just a quick comment reflection after reading the article and mulling it over a bit - surely more is to be said on such an enduring topic.