The lack of funding angle isn't really convincing.
Modern designs depend on material science and computing abilities which could never have been made in the 70's no matter how much money was thrown at it.
Fusion-relevant materials research could have absolutely advanced with funding back in the 70s. Lithium compatible structural material and 14MeV neutron source experiments immediately come to mind, not to mention tritium permeation and extraction. There was tonnes to learn, and they chose not to fund it.
Modern designs depend on material science and computing abilities which could never have been made in the 70's no matter how much money was thrown at it.