I think it would be major news wrt our modern understanding of the brain if it turned out that we indeed should learn fewer things rather than more things to keep our brains functioning optimally. That sounds like 1950s superstition all over again, like running out of neurons.
I think if I had linguistic skills tested, I would probably be rated above-average for both English and my maternal tongue, but somehow to me it just doesn't feel that way... and I find this profoundly annoying, I feel home nowhere.
I don’t know what I believe is true - I’m just saying it’s possible the study could be true AND it also be the case that many multilinguals feel the opposite
I don't think this is true. Unless you've misreading the Dunning Krueger paper.
It seems consistent with "every answer coming with two more questions" a "tree-like" question structure as knowing more means knowing more about the unknowns.
So someone less familiar would think "Carpentry? Illiterate innumerate peasants could do it, more strength than skill." Meanwhile even a master carpenter of bespoke homes would know that they are inferior to a jouneyman cooper who worked before metallic barrel rings because they didn't have call for making joins that round and tight, that they hardly even worked with steam bending, and that they use automated tools that millenia ago would have been done by say hand and feel and calculations rather than using a micrometer.
One could argue that such events occur for everyone, but for a monolingual it would speak to the failings of the one language they speak, not of their inability to express thoughts. Or maybe such events occur more often for those who are bilingual because the brain can come up with a greater diversity of thoughts, owing to its mastery of two "modes" of thought.
Looking at the numbers effectively it seems that monolinguals have diminished returns - and I can see that just because a lot of problem spaces are like that.
Granted "vocabulary size" isn't quite apples to apples given how languages and syntaxes differ.
It's not an absolute rule, and not something that has been (or can be) proven.
So if you're ready about a study that supports common sense, it had unfair advantages to reach you and is less likely to have underlying merit.