The gel isn't "churned". The path you see carved out is where the bullet has cut through. There is also going to be variability in how he slices the halves. You'll notice he uses that little pick thing to pry open the damage channel so it can be seen better.
Why do you think the maximum cavity isn't produced until several inches in? Because the bullet hasn't fully expanded yet! The damage is only being caused by what the bullet is physically touching. If temporary cavity/energy was causing damage you would see more damage at the entry point because that is where max energy is occurring but all you see is a wound track the diameter of the original caliber.
BTW, his .40 180gr HST test was one of the best for wound channel, in any weight or caliber.
What more proof? Look at one of his few videos where he shows front facing cross-sections of gel. The only damage is where the shaft of the bullet and the petals have cut through. For example, watch this starting at 3:30:
BUT... the .357 Sig videos do show some evidence of the temporary cavity actually permanently deforming gel outside of the permanent cavity. So anyone dissing the .357 Sig hasn't been paying attention.