"Toxic" and "Delusional" aren't the same thing, if a poor bastard thinks Zoro would defeat Luffy in a fight they are delusional but if they respectfully state their opinion and move on, there is nothing toxic about that. ZKK is not delusional, the possibility is there, personally I don't believe the possibility is high but it is surely present, but the toxicity comes with the cult like push; when it's becomes a us vs them thing, where anyone who disagrees is labeled a Zoro hater, stan of another character or said other character is attacked; when the same theory with the same points is spammed every chapter with the aim of drowning out any other topic and when said people put their opinion on Oda as a writer on whether or not it happens, then that is definitely toxic; no one active in the community can deny that this is very common behavior among the ZKK enthusiasts.
As for your claim that the majority shittalked non toxic Zoro fans saying that Zoro will fight Kaido, then they themselves are toxic, but it's important to know how that idea was being pushed, in what manner, was it aggressive?.....if yes then the reply is understandable, if not then like I said before it's toxic.