Updated first post with some clarifications on certain S ranks.
Unless you have complete control of who is in the zone and who's currently eating or making food in the zone, how do you know? On Excalibur I would misdirect the masses of people to go somewhere else (usually for an A rank, which gets their attention) while a closed group of people launch themselves at a specific zone, executing my theories in a controlled environment once we see the zone to have been sufficiently cleared out. Because we know for certain that when a trigger causes a spawn, somewhere in the zone a mob will pop, we can be at every spawn point and attempt to do the triggers to see if something happens. The "hotspots" (spawn points) are not necessarily going to pop out a mob next to the person doing the trigger at the time, as we often see the S ranks during controlled tests pop out on the other end of the zone opposite of the person currently (hopefully) solo-triggering the current test.
I am not doubting your findings outside of wanting to know if you have had a controlled environment to test it in. Using vendor food has not been actively tested on Brontes in particular since we don't know for certain yet when to start trying. My focus has been directed at things like Wulgaru, Garlok, Safat and Nunyunuwi for the time being while we boggle our minds at how they work.