I see what you mean. Thats just Jeffro. He was afraid his plane would be too light.
If my dowels were longer than about 2.25" I might cap them with some 1/8" aircraft grade ply but really the engine back plate and the firewall do exactly the same thing. True torsional stiffening would be shear webs between the dowels or angle braces to distribute torsional loads.
I just use the dowels. My 115 powered plane is on 2"x7/8" dowels, no bracing, 2.5 years of steady flying. I evan cut a large hole in the center of the firewall between the dowels to drop some weight and no problems. One other thing I do is to use a hole saw to make aluminum washers to cap the dowels. Metal washers work but aluminum washers cut from the same sheet have identical thicknesses.