The problem here IMO is that although intelligence can be fairly well defined and measured and it tends to be fairly static over a given period of time, happiness can be more difficult to measure or define and fluctuates.
Someone may say they are less happy on Monday Morning than on Friday afternoon, or they were happier last year but then the pandemic lock down saddened them. And, some people perceive their own happiness differently, on person may say, I am very happy, but they perhaps are not as happy as another person who says they are only moderately happy because they may have a higher expectation of happiness.
And what is it really, many philosophers have spent a lot of time on that topic. Is it that all my needs and wants are met and I am in a committed relationship? Others may define it differently. The article touches on these points.
In my view these are just two different unconnected attributes, asking if intellect and happiness are correlated is like asking "how big is the color red?" it is not really a meaningful question.
Anyone can be happy, whether they have a high IQ or not.
Say you have one man who has a low IQ, works as a laborer, but comes home to a wife and loving children. They do not have a large house or fancy car but if you ask him, he may say he is very happy.
Another man, lives alone in a large home with a nice car but enjoys solitude and contemplation, perhaps occasionally having a friend over and they have a deep conversation about quantum mechanics. You ask him and he might say he is very happy.
Obviously, you could easily reverse those examples and add many more.
Now let's measure that?
Personally, I just don't see how.