I've tried to go through that trapdoor in Zulag One countless times, but have failed. The way I see it is that this person might've just edited the video to make it seem as if they went through. I think the reason why there's a bird portal there is because, when you return to Rupture Farms, that trapdoor leads to a secreat area, and I believe the bird portal is located somewhere around where the video showed it.
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