I may be misunderstanding you but I don’t think you have the explanation quite right. Pumping is most effective when pushing your legs down (standing up in your terms) as you descend each ramp, not before. If you just stand up on the moderately level bit at the top you don’t get nearly as much effect.
The key to pumping is to pump on the down and also to pump just prior to each up.
This video has a pretty good explanation I think.
Note also that when skateboarding a big half pipe, you’re standing when your body is nearly horizontal, it doesn’t add potential energy by making your centre of mass higher, it adds kinetic energy by propelling your body horizontally in the direction you want to go.
Edit: And that is why being strong makes a difference. You’re launching yourself forward’s off the down ramp, and up just prior to the up ramp.