crazyrabbits
Banned
SPOILER
To make it short, the parasite infects the entire Mother Base and Big Boss kills everyone to avoid the parasite to spread over the world.
Whoever wrote the guide believes this is the turning point for Big Boss, which makes this Big Boss the real one and not Venom.
That said, I don't know if they're aware of the secret ending that (should) reveal Venom's true identity, so.. I don't know. Mother Base will be destroyed again, that's sure.
When they say "kills everyone", does that mean BB kills off named characters? It seems Quiet isn't there at that point in the story (she apparently leaves for some reason, turning up as a boss battle in Mission 43, according to the guy who posted the scans/videos), and I doubt Huey/Ocelot would be there, but do they really state that you have to kill all the soldiers you spent time extracting?