As far as whether or not it's Big Boss, I would say so. If it was just the GDC ad, that would be one thing, but the other piece of art matches up with the screenshot they released a few months ago. Just makes sense, I think, especially with how much attention he's gotten from KP the past few years.
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"When I was a young man, I had liberty, but I did not see it. I had time, but I did not know it."