I did a lot of research on this [1]. I got confirmation from Robert Garner (architect of the SPARC processor) that the NSA did indeed ask for the population count instruction. His story of meeting with the NSA is pretty amusing [2].

[1] https://retrocomputing.stackexchange.com/a/8666/4158

[2] https://archive.computerhistory.org/resources/access/text/20...