I thought the first atomic bomb proved his theory?
Apparently no measurement is official with the French signing off on it.
All sorts of nuclear interactions prove it. The kinetic energy plus rest energy is conserved when particles collide and interact to create new particles.
The idea here is that the calculations account for the rest mass of neutrons and protons in terms of the interaction energy of quarks and gluons. This is more a validation of quark theory than a proof of E = mc2, IMHO. The latter is amply demonstrated by the annihilation of a proton with an antiproton.
Lise Meitner, a female Jewish physicist, proved Einstein’s theory, much to the dismay of the Nazis. She was working at the German University that took the credit for it. She had to be smuggled out of Germany before she could finish her work. She finished much of her work by mail.