I’m pretty sure all that asker was saying with the “end of book 2” was that it’s the end of the S9 arc. I don’t think it has any more implications than that.
The thing is, I was expecting a boss battle and an Interlude Arc. To be fair, the Nine have been a three-Arc-long boss battle of sorts, but I was thinking we’d be seeing another Endbringer attack fairly soon.
But if an Endbringer Arc, followed by an Interlude Arc, were to show up within the next three or so Arcs, people would not consider that the end of book 2, not Prey.
(I’m 99% certain more Interlude Arcs will happen at some point. Especially because I have seen how Sentinel looks in the official table of contents.)