Physically demanding doesn't necessarily mean heavy. Moving at a rapid pace for extended periods is considered physically demanding.
If you're in regular shape at 50 or 60, this is tiring but absolutely doable for a few days or weeks. I'd even argue that it's healthy to physically push yourself.
But someone else noted that these temp workers aren't expected to hit the same output.
Amazon warehouse jobs (and I suspect it's hardly unique to them) are pretty universally described as intensely physically demanding.