Mr mooney, I dunno, I cant imagine that a fairly new deore rd is slack. Heck, I have a nearly 30 yr old one on my old touring bike that is still good and snappy, and have a 9 or 10 season old one on one of my bikes, same thing.
I also dont know if you can change this spring tension, like on older v brakes with changing the hole the spring goes into.

it seems to me that in the big big photo, the rd isnt as far forward as it is when I do this funky test on my bikes, which I really dont like doing....which would point to the chain being a bit too long.

if his chain is a shimano, it prob wont have a quick link, so probably not encouraging to do a chain length test.

who knows, maybe this was a replacement chain put on without measuring, and put on with all the links on it in box.