That's ludicrous nonsense. The primary breakthrough of the theory of relatively was taking what the math had been telling physicists about the universe for years seriously. In hindsight, everything necessary was there before Einstein, it just was very deeply assumed not to be reasonable. Relativity builds on numerous pre-existing foundations and is very much in the "standing on the shoulder of giants" tradition.
https://en.wikipedia.org/wiki/Theory_of_relativity#Developme... , first paragraph
Either you've misinterpreted Adam Frank's work, or it isn't excellent.
I'll counter-cite Reflections on Relativity, which contains a lot of deep analysis of the mathematical history of relativity both before and after Einstein: https://mathpages.com/rr/rrtoc.htm which makes it quite clear both what he did, and did not do.
That's the sign of an expert, not a generalist.
But he wasn't ignorant of them. He knew the existing work. He was extremely well read in physics and maths. He wasn't a generalist!
If you get a group of random generalists to build your compiler, or your graphics engine, or your cryptography stack, they aren't going to know what they don't know, and it'd take them decades to come up to speed.
The amount of unique stuff in each subfield is not that great, and if you have a wide and solid base in different subfields, picking up the unique ideas and techniques isn't that hard or time consuming.
But after a generalist has it, he is a lot more productive and creative because he often applies stuff from other subfields to come up with better/novel solution to the problem at hand - something a specialist cannot do.