Freedom = $1.05
Banned
They already explained it in MG2 by speculating that Big Boss had majority of his body replaced by mechanical components. Considering the crazy bionic arm shenanigans you get in MGSV that actually seems the more obvious and plausible resolution than a body double.
I thought i remember something about BB in MG2 essentially being a Snatcher but i don't recall where i got those details from.
Maybe I'm forgetting something, but I remember both of those being his decision. He gets the final say on Battle Gear going into development, and Kaz grills him over bringing Sahelanthropus to the base in a tape.
I thought Sahelanthropus coming back to the base was something Venom did grudgingly but maybe i'm misremembering