I boarded the shuttle at NAIA 1, where it is landside, after arriving on OZ. The drivers did not check BPs and seemed to have no concern for which airline anyone arrived on. They asked what airline people were leaving on, which I had assumed was so they would know where to let us off. I was connecting to domestic on Cebu Pacific in T3 on a separate PNR.

My comments about "airside" I should have explained better. The boarding at T1 was landside. The shuttle then drove on the tarmac past T2 and to T4, where if I recall two people got off but no one got on. When we arrived at T3 the shuttle stayed airside, we got off and entered the terminal through some back hallways, and emerged after a short walk into the terminal landside near the checkin counters. Sorry that was not clearly explained in my first post.

So maybe at T4 there is a similar way to board/exit from landside despite the shuttle being airside, but I wouldn't know personally.