Which is not at all unrealistic, because POPCNT had been recognized as useful and it had already been implemented in some of the earliest computers with vacuum

Which is not at all unrealistic, because POPCNT had been recognized as useful and it had already been implemented in some of the earliest computers with vacuum tubes (at the suggestion of Alan Turing). Those early computers, like the Ferranti Mark 1, were much simpler than Intel 8086. The name "population count" had been introduced for this instruction by Cray 1, a couple of years before the launch of Intel 8086.