A is trying to dazzle you with brilliance, but if that fails he is falling back on the baffle you with BS. Regardless of what happened in the past, and he is just guessing, unless it is a problem now there is no need to redo it. If the need should ever occur it can be done at that time. Second, there is no cross connection connecting to the garbage disposer, if it is done correctly, and if there were a problem, connecting to the sink drain is not going to correct it. Wait for B or contact C.