Although you engage more muscles, the other muscles are being used less. You are moving some of the load from your legs to your arms and your core. For the same power I can stand and drop my RPE and HR.
Me too, for RPE, though I'm surprised about the HR. And of course I agree about the arms and core thing, which accounts for the RPE - more distributed loading across more (and different) muscles. But I still think the link is right about higher energy consumption overall. If we took a 1 mile climb at the same pace on successive days, standing one day and seated the next, which do you think would have the greater impact on the rest of our ride? In my case the standing mile would tire me more. You may disagree. If so I'd be interested.