A more accurate way to say it is, “if you tolerate the intolerant BEING intolerant, intolerance will eventually wipe out tolerance.”

If the intolerant could mind their own business and tolerate people they didn’t agree with, they literally wouldn’t be part of the intolerant. That’s the point: it’s a core part of who they are and we have to cut it out like a cancer to have a tolerant society. (Sorry for making you read the T-word so many times.)