The way that the report functions are written, alongside with the terms of service/code of conduct, is to be vague where it is “generally applicable” unless it has been specified.
Someone stating that they will not tank a boss unless someone gets kicked would definitely quantify as gameplay sabotage. Yet you won’t even entertain that notion because this notion is far more logical than to assume that acting with an explicit intention to sabotage the group (“unless you kick them I won’t do X”) doesn’t constitute as gameplay sabotage.
Yeah… somehow I think it is pretty ironic that you are the one to say that someone is unable to entertain other points of view when you’d rather, with no information except what OP provided, insist that this was a case of someone being incompetent.