Well, I couldn't replicate it, but I just activated the first phase of the boss, left the boss room, went back and the boss was still active (I could start the next "attack"), but the barrier was gone and I could just finish the demo. I'm not sure, but it could be related to quitting and reloading a bunch of times on the same save before the bossfight. I also visited the out of bounds section.

Anyway, I found another way to practically skip most of the boss. After starting to scan the big bird during the first phase, it will normally push you off the platform, but if you start scanning again while being pushed, you'll just clip through the bird. If you already knew about that and decided not to remove it, I think you should make the timing more precise or make the strat harder in some other way, because I actually found this on my first playthrough and it feels way too easy.

Edit: I actually switched from Past Simple to Present Simple in the middle of a sentence and then went back to Past Simple, it should be fixed now.

Edit 2: Wow, that's a ton of text in a single comment.