I think it was easier in that era; AFAIK they used conventional secrecy methods (project names, locations, misdirection, need to know, obfuscation) to pull it off. Feynman's "Surely you're joking" and Rhodes "making the atomic bomb" are good sources for some examples (and otherwise recommended)
There is a consensus among most people I know that if top researchers are necessary for the successful development of AGI, then it would be impossible to do in in secrecy, because the world would notice top AI researchers leaving their jobs and congregating at a military facility.
But the US government did pull this off with physicists. How?