This is an old video, probably over two years old. I am sure Joel has moved on cutting steel with an angle grinder. I would guess his blanks are laser cut these days.
Why people use angle grinders? Because water or laser jet cutting is expensive if you buy steel by a sheet. Say 80LB sheet will yield about 80 knives. At $10+/cut, you shelling out $800+ extra plus shipping. Take PM steel at $21/lb, add shipping to the laser/waterjet cutting facility, cost of cutting and return shipping and you start seeing a real incentive to cut steel yourself by whatever means (including angle grinder) available.
I use angle grinder and a portable bandsaw at the moment. I might get a stationary bandsaw, or use laser jet or waterjet cutting service next time I order, but upfront cost is high in both cases.
Sharpening before finishing? Now, that is something worth to wonder about. As honing an edge on slack leather (razor hone).