Here in California, I’ve heard both “military time” and “24 hour time” used interchangeably for writing the time as “03:45” or “16:20”. That said, I’ve heard – citation needed – that proper military time does not use the colon, such as “1600”, pronounced as “sixteen hundred hours”.
As for why the public might refer to this generally as “military time”, it may just be that that’s the most common, well-known use-case in the States, outside of the sciences. I personally use 24 hour time on all my devices, but I’ve come across many people who prefer clockfaces or AM/PM, probably out of habit.