However, because its a forward facing stance, the muscles involved aren't really related to movement on rock.
Though he is pulling not pressing. I wonder what exercise you could do to strengthen those hamstrings?
Just go running. No need to get too technical.
This is what Athletik Spesifik (late of this parish) recommends:http://www.athletikspesifik.com/climbing-circuit.html
Recently I've started to notice just how weak my legs really are, and now I'm thinking about it more whilst climbing I reckon it's affecting me a lot more than I'd realised before.I've been trying to build them up recently with the eventual aim of being able to do a pistol as that seems to be the most extreme leg movement used whilst climbing. I'm not having much joy and can barely do the 'Method two - single leg step-ups' on a 1 foot box as shown on http://www.beastskills.com/Pistol.htm
Its another one of those tall person disadvantages IMHO :-)