Rumor was that Edison couldn't understand how one could
measure 120 volts between any two of three terminals.

No rumor about it. Westinghouse published several papers on electricity
where he described voltage and current phase and had a true understanding of
it. Edison was a hands-on experimenter who had little theoretical physics
or mathematics background. That's why the two had a feud about AC vs DC.
After Edison's assistant died of radiation poisoning, he was very leary of
things he didn't understand and tried to convince people that AC current was
just too dangerous. He also refused any more experiments concerning
radiation.