I've used a post hole digger to dig out holes for a fence and a shovel to dig large holes for a septic system. I would never use the post hole digger for digging the large cascading holes, one flowing into the other.
It just doesn't make sense. And I'm sure someone in the landscape business would use the proper tools for holes.
Well, he denies he was making a fence for anybody, you figure it out. Ricci being a professional would probably use the 2 handle type where you force it in the ground and spread the handles apart to lift the dirt out and this would explain how Moul knew what it was from a distance. Also Ricci may have borrowed the post hole digger from his company or someone and needed to return it whereas he used his own shovel and dumped it on the way back.
You are assuming that Ricci knew that he would be burying a body. If his accomplice killed ES, which would account for Ricci's extremely sour mood when he returned the Jeep on the 8th, he may have had to use the tool(s) he had available.
I don't have any love for Ricci, but my instincts are that he wouldn't have been party to a plan to kill her from the get go.