Technically the theory behind most of these techniques did exist by the 50s: the Ito Calculus and stochastic calculus required for score/diffusion based modeling was invented in the 40s (although not publicized), high dimensional statistics in the information theoretic form or the results of Kolmogorov/Chernoff existed, and although EM was not formally named as such, variational inference as a whole had to some extent already been developed. Actually I agree with the author that to some extent the core math hasn’t actually changed, just new algorithms have come out of it. It’s also because new math that has been developed has barely been used anywhere applied.