I believe you’re correct that there’s no physical pathway because Barathus would fly up. The biggest clue for me is if you look at the bridge magnified area, you can see at the top of the bubble the faded hallways and top of the integrated science lab.
My biggest wonder of this Book is why there’s no map for the control room of the Stewards complex. I get that maybe they didn’t want to draft another larger compound after Book 3, but if that’s the penultimate and climactic event with the Stewards directly l, it feels glossed over that there wouldn’t be even a small map. My thought is the real battle is at the biotech company but there’s no way for the players to know that. Guess I’ll just make one myself!