I think the P&W claim here is just plain wrong.
757-200 Operating Empty Mass:
with P&W Engines - 57,840kg
with RR Engines - 57,975kg
So each RR engine weighs 67.5 kg more than a P&W engine. Pretty irrelevant if you ask me.
And just to be sure, in case the claim refers to the 757-300:
OEM with P&W Engines - 64,460kg
OEM with RR Engines - 64,590kg
Even less of a weight difference....