RETE family algorithms did turn out to be somewhat parallelizable, enough to get a real speed-up on ordinary multicore CPUs. There was an idea in the 1980s that symbolic AI would be massively parallelizable that turned out to be a disappointment.
You could argue that since automatic differentiation and symbolic differentiation are equivalent, [1] symbolic AI did succeed by becoming massively parallelizable, we just needed to scale up the data and hardware in kind.
https://en.wikipedia.org/wiki/Fifth_Generation_Computer_Syst...