The brilliance and drive he displayed, as a child, would be a good place to start. But, I'm a believer that knowledge is limited by dedication and intelligence is limited by nature. This belief came after I suffered from a 6 hour TIA, that passed through my cerebrum, causing me to lose some of my intelligence. This was a wakeup call that made me realize that the ease that I had before wasn't my own accomplishment, or somehow related to my efforts.
Temporarily or permanently? I'm curious, because I've likely had one too (much shorter though). I definitely noticed my inability to think properly. Medium-term memory (not sure what it's called, but remembering anything past a dozen seconds) was also limited or non-existent.
But, I had pretty great success with hyperbaric oxygen therapy [1] in combination with oxiracetam [2][3]. I would highly recommend looking into it, and it might be covered by your insurance. I was really struggling with work, and it definitely helped that, my lack of motivation, my muted personality, and some anger issues. My wife says she noticed a big difference and claims I'm now back to normal. I would say that I now can think in short paragraphs, which is sure a lot better than before! My brain is my job, so the cost was easily justified. It really was a big change in a short amount of time.
The weirdest and shittiest lingering problem is that I have apparently mostly lost the sensation of needing to breathe. Before the oxygen therapy I could just hold my breath and literally not notice. It took me passing out a few times to realize that I was forgetting to breath during conversation. This made exercise difficult. After the oxygen therapy, a slight urge to breathe has come back. I can exercise, and I'm not afraid to swim now, but if I'm deep in thought, I'll forget to breathe and end up feeling pretty terrible. I plan on buying a continuous, wearable, oxygen monitor...
[1] https://www.sciencedaily.com/releases/2013/01/130123144218.h...
[2] https://pubmed.ncbi.nlm.nih.gov/31057392/ Note that the oxiracetam was my thing, not prescribed by the doctor (he did approve of it though, as long as I was taking some choline supplement). I now take ~1g/day with 200mg centrophenoxine (choline), and fish oil on weekdays. This seems to really help.
[3] Can't find the exact NIH reference for this one that I really liked, but I'll reply when I find it. But, the concept is (my understanding) that capillaries grow where oxygen is needed. A TIA will tremendously clog capillaries. The neurons that were deprived from the clogged capillaries don't receive enough oxygen to metabolism enough to promote new capillary growth. So they stay "dormant". Promoting capillary growth and waking up these dormant ones is how someone with a stroke really benefits. This was all echoed by the doctor that reviewed my situation before the therapy could start.
I'm not denying talent, just stating that in this context (rare advances) I don't think it's the main limiting factor.
By the criteria that were in effect at the time, which weren't that different from the ones in effect now. Einstein was working as a patent clerk when he published his 1905 papers, but patent clerk was just a day job, and everybody who mattered in physics knew that. He already had a degree and was working on his Ph.D. (which he was awarded soon after). He took the patent clerk job because he couldn't get a teaching post after getting his first degree, because none of his professors would give him a good recommendation; but he still had plenty of contacts in physics. His 1905 papers were published in Annalen der Physik, the most prestigious physics journal in the world at that time, and that wouldn't have happened if he'd been just some random patent clerk.
While this is true, I think it's also true that the ability to be frivolous even about big questions like the fundamental laws of physics is rare. It's egalitarian because frivolity is fundamentally opposed to any kind of elitism, not because frivolity about fundamental questions is common.
To champion the idealists for a moment, what's the practical implication of realism in this case? Elitism in retrospect is one thing, but trying to bring it forward or explicitly advancing it seems like it would result poorly. By what criteria would an Albert Einstein, the patent clerk, ever have been selected? Your best bet at an Einstein is to let everyone in and see who discovers relativity.
It seems to me that the only way to enrich the elite is to empower the masses, so to speak. In any case, I think it's more important to focus on what circumstances allow a person's potential to emerge. Potential is a highly available resource.
Like the author note, the problem isn't that most will fail. It's that most will never try, tenure or no. The free mindedness required to even attempt greatness is often frivolous. Frivolousness is more of an egalitarian than an elitist hallmark.
*Note: I'm not referring to politics or economics in any way. Just playing along with the language of the article.