No, they mean that they used various biographical accounts of their behavior to make that assessment. So inaccurate as %*%*, BUT I have no doubt TJ was brilliant. He was youngest member of Congress when he was selected to draft some of our founding documents.
So I’m a modern day graduate of the College of William and Mary (picked up my JD there) and consider myself pretty bright. The lowest I’ve scored on various grad school entrance exams still placed me in the top 98% of testtakers. I speak two languages conversationally and can halfway fake my way through a third enough to get and give directions and other touristy things. I can usually understand and engage in wide range of subjects from physics to chemistry to whatever (In fact, in law school I made my spare cash doing freelance ghostwriting projects covering topics in magazines devoted to astronomy, legal issues, physics, palaeontology, biology/wildlife management, photography, etc...). I still play the sax in my spare time. I’ve got numerous hobbies like creating home brew retro games for vintage consoles and computers to literally home brewing booze.
So I’m pretty smart....TJ is a full on genius. He spoke SEVEN languages fluently as opposed to my 2.5. He was considered an excellent player of five completely different musical instruments while I’m merely a good player of one. He invented countless machines (which he chose not to patent as he believed advancements should be shared freely) including the earliest polygraph machine, one of the first portable printing presses, the revolving door still used today, the first dumbwaiter/elevator, many different revolving cyphers used for coded conversations for military and political purposes, etc... He was one of the first people to seriously study agriculture in the US including being one of the first white individuals to farm tomatoes (considered poisonous and inedible before him), developed strains of rice that made America one of the highest rice producers, developed grape hybrids that allowed so called noble grapes to be grown in the US (there’s a root rot disease that kills any varieties straight from Europe being grown east of the Mississippi) so that good wine not just sweet muscadine wines could be made in the US, etc... He studied books on architecture for about a year and then personally designed his plantation home in Monticello, Virginia and not only has it survived centuries, but is considered one of the most beautiful buildings in the US and is still copied to this day (FSU’s law school is based on its design). He redesigned the roads in D.C. and became basically one of the first civil engineers in the US as most cities were not planned prior to this. He spoke modern Greek fluently and studied Ancient Greek and as a result created the Jefferson Bible by translating the oldest Greek sources available into the English of the time. The University of Virginia was created by Thomas Jefferson and its original library was just TJs donated library (every single book was said to have been read by TJ at least once and covered every subject Universities would have been interested in studying).
So yeah...TJ would have had no problems succeeding in the modern day. He was a literal genius and blows my tiny accomplishments completely out of the water.