Eliminating or reducing government subsidies for oil and coal companies probably would have a climate impact. Even if divestment has “zero impact” as Gates suggests (which I doubt, but ok), it would have a symbolic importance and possibly spur others on to change their approach.
Do the symbolic with your money, I have respect for that.
But a bunch of about to be retired people, who may need every penny of their return, will get reduced return because some fund manager wanted to feel good and have better PR all the while collecting the same big check for his services... No Bueno!