I learned two languages growing up and was speaking both as soon as I could speak and could write in both not long after. This is typical for nearly every kid in the world outside of countries with strong language monocultures. I certainly think Mills was a very talented person, but there's this weird cult of being impressed by "speaks 7 languages" hagoigraphies which aren't helpful. People bring up it as some acid test of intelligence and its just not very accurate.
Especially when you actually know the language these kinds of people claim to speak and you realize they actually don't speak 7 languages but maybe know 2 or 3 fluently and know 'kitchen' versions of all the others. I'm not going to name names because I don't want an argument and don't have the spoons for it, but lots of these international luminaries and leaders and such with "speaks 7 language" are often little more than conmen or simply enjoy building their own little hagiographies for their own PR goals.
There's this wonderful deep-dive on youtube on Feynman's high-questionable personal mythology that is a great example of this kind of self-promotion and how easy it is to sell one's self, especially in academic and techie circles, if you have a certain amount of charisma and drive.
Also as a lefty, I'm also not impressed by breathless ambidextrous tales either as most lefties are forced to be ambidextrous and its not actually exceptional at all. I can write with both hands, play musical instruments either way, play sports either way, etc. The left hand is better at these things, but my right-hand is okay-ish at almost all these things and I use a right-hand dominant near everything in my life anyway. I even like to switch it up to keep wear and tear down. At work the mouse is on the left, but at home for gaming its on the right. This is all boring everyday stuff for lefties.
There's a toxic 'great man' mythology that humanity still can't get over and its weird seeing it taken seriously when so many 'great men' have been debunked or seen as recipients of the system they were under (Mills' father pushing him so hard and being in the privileged class that would allow all this instead of back-breaking farm labor all day). Personal talent is important but its vastly played up in dishonest ways for dishonest gains. We probably pass many highly talented people a day on the street, but only some had the opportunity to grow those gifts into something they can use.
The famous quote comes to mind. "I am, somehow, less interested in the weight and convolutions of Einstein’s brain than in the near certainty that people of equal talent have lived and died in cotton fields and sweatshops." - Stephen Jay Gould