Sorry, I'd much rather not get into this too deeply once more so I'm going to go for a much simplified version (the expanded version is in the original post which you've said you've read and usually this sort of question can run on for multiple posts/pages which I would honestly rather avoid) if you don't mind (but if greater detail is wished for I will continue to try).
Basically, every Booker could only be drowned if a single Comstock could accept the baptism. This part you understand I presume because a single Comstock has to exist for Elizabeth to exist to drown him (because it is only through Comstock's actions that Elizabeth can exist to murder Comstock before any of the chain can occur; a paradox). I hope you can also see why this is a paradox since if you've that part down it makes explaining things much easier but if not the simplified version is because Elizabeth's existence occurs after Booker/Comstock would be dead by her hand. However, Elizabeth drowning every Booker would cancel out itself, because a Booker must accept for Elizabeth to do this at all.
Therefore:
Booker accepting = Paradox where Booker died before accepting. Probability of paradox, and thus acception = 0. If the probability is zero, the timelines/universes never exist.
Booker rejection = Nothing. No paradox. Probability of other action = 0 therefore probability of this action = 1. Probability of 1 is a constant, so it always happened, will happen, and happens. Booker never accepted the baptism, ever, the game's events don't exist, no Booker literally dies in the game's ending.
I don't know if a Venn Diagram version will work better since it's the only thing timeline-like thing not in the original post related to the ending I've seen in the thread and there's no point in repeating the others (better ones) so here:
Each subset occurs after the set it belongs to. The left hand side is what would happen if the game's events could occur and the baptism wasn't a constant. As you can see, if any set of Booker accepting exists, there is a paradox where it is cancelled out. It's only cancelled out by a subset in the acception timelines, no such cancellation originates from the reject timelines. If only the blue set exists, if Booker can only reject (and the baptism is a constant), there isn't anything to cancel it out.
If it's hard to imagine precisely why (since it's a very abstract concept) then just go with only the accept Bookers being drowned since it has the same result and is easier to understand.
EDIT: I hope this makes it somewhat clearer/easier to follow; admittedly I'm sure it is possible to explain why in a more concise format but unfortunately I'm not really able to