: How do we know the game action depicted, is from normal retail content?

If you read his description, he states clearly that he's turned infinite ammo (and invincibility) on to speed up the opening section, so that he could show you the actual egg without having to make you watch an hour-long battle first.

It's very obviously NOT normal retail content - he never tries to claim it is. He's modded the game in multiple ways to make the display of the egg simpler. (I am taking at face value the implicit assurance that none of these mods have made the egg possible beyond what the normal retail game could do.)