"Drink the Kool-Aid" comes from the 1978 tragedy at Jonestown, where, at the urging of their spiritual leader Jim Jones, over 900 people drank cyanide-laced Kool-Aid and died. Since that infamous event, the phrase drink the Kool-Aid eventually came to mean to follow someone (or some school of thought) blindly, without question.
Therefore, the word multiculturalism here is just a qualifier; someone could talk about any "flavor" of Kool-Aid they refused to drink.
I've heard this expression used when something was gaining cultural momentum, and one individual wanted to avow that he would not get caught up in the hype. As an example, say that local sports fans are getting excited about a relatively new coach in town – one cynical fan might say, "I'm not ready to drink the Harbaugh Kool-Aid," meaning roughly, "I don't think everything Coach Harbaugh says is necessarily correct."
Conversely, I've heard "drank the Kool-Aid" used to express that someone was ready to take someone else at their word:
"I'm not sure that was such a good trade our team made."
"I don't know – I've drunk the Harbaugh Kool-Aid."
That dialog implies the second speaker is impressed with Coach Harbaugh, and prepared to give him the benefit of the doubt, even if he does have some misgivings about some of the decisions made by the coach.
Whatever comment you made in your debate, someone thought it seemed too "multicultural" (which was perhaps another word for "politically correct?"), and so they used the Kool-Aid reference to express their disapproval, ostensibly on behalf of the rest of the regulars in that forum.