Well they probably are using the fact that when you are in spectator and spectate an enderman, creeper & spider you get a special shader applied. There is also a bug in the game where I think if you kill the player in spectator while spectating one of these mobs the shader does not get removed, you can even switch gamemodes and still does not get rid of it. pressing f4 does toggle it for some reason. I have not taken a look at the map but that I think that answers your question



edit: you can use /spectate used as: /execute as @s run specatate @e[type=creeper,tag=tag,limit=1]to automate the process of spectating mobs, and so they do not have to click the mob to spectate it.

I noticed that when the shader loads in for a split second a creeper spawns at your location. Is this part of it?