There is no line. It's a spectrum. Milsim is simply an adjective, and things can be more milsim or less milsim, just as something can be more blue or less blue.
So lets take a Dye DAM. Take off the stock, take off the foregrip, pick one in a more flashy color, and it's considerably less milsim. With it still being mag fed it's still slightly milsim, but only a bit. Run it with a stock, foregrip, add a few tacticool accessories, get it in a camo pattern, and it's way more milsim. Now it's not a 100% exact replica of an existing firearm, so it's not "pure" milsim, but it's getting up there.
This can then apply to anything. A Tippmann A5 is a bit more milsim than a standard Model98. A plate carrier is more milsim than a Dye Tactical Assault harness, but that's more milsim than a regular pod pack. A sci-fi themed scenario game is obviously not pure milsim, but it's more milsim than a speedball tournament.
Nunquam emere quid potes facere