Milsim is (or should be) about realism.
It's not necessarily game length or complexity.
It's a player mindset where the BBs can kill, rather than a minor COD-style annoyance you'll respawn from after a few minutes.
It's a realistic approach to solving a given problem, where shooting is only one of many tools at your disposal (and not always the best tool to use); spraying off hicap after hicap is not an option (and realcap should be the way to go).
It's about realistic loadouts, etc.
It's about realistic operations.
Just about any game/scenario can be made milsim: the succes of milsim comes from the players not the organizers. An org can do everything to set up a milsim and people will just run around and shoot like idiots; similarly a simple "skirmish" can be milsim-as-fuck if everyone on the field can get their shit together and elevate their game.