He's using an Azurewrath, which is a great weapon for the build but not as good as Sun Keeper.
The WD Reddit community has this
great spreadsheet for calculating Harvest damage; if I have all of my buffs going I can get my Soul Harvests to do ~613m (which includes getting Strongarm to proc and Piranhas; without Strongarm (e.g. any Rift Guardian) it is ~515m.) For whatever it's worth, if I had perfect rolls on Strongarm proc, Cold damage, and Elite damage it would peak at 705m with Strongarm proc and 559m without it, so I have a lot of room for improvement.
If I pretend I have fender's Azurewrath, those numbers change to 483m (with Strongarm) and 405m (without Strongarm). Azurewrath
does do better on random trash, but random trash isn't really any issue in the first place. fender might be getting slightly different numbers because he doesn't have the same Cold damage, CC, or CD numbers that I'm plugging in for me but the basic principle should be the same.
TLDR: Get a Sun Keeper with a high roll on elite damage.
Edit: Even if you don't have Strongarm, Piranhas is still worth +15%.