Sorry, where's the contradiction? Vivzie says:
"The way we see it is, these two universes are connected, but they almost feel like their own realms, which is how characters can portal between them."
Then follows it up later with:
"like two different planets in a lot of ways — that's kind of how we treat them."
"That means Heaven is physically accessible from Hell, not just via portals, but by traversing physical space."
The literal description is that they are two universes that are connected, and are like planets which is why they can be travelled physically. I don't see any contradiction at all.