What Reese is basically saying is that in addition to the trailer SAFETY chains attaching to the extension ball mount, you need an additional set of SAFETY chains to attach the extension to the truck receiver, for safety continuity.
These SAFETY chains would be installed loosely.
I've gone one step further, and welded a tab on each end of the receiver, then run a tightened chain/turnbuckle setup from end of extension to receiver. This helps keep in check the side to side forces, and functions as a safety chain.
'01 Dodge 2500 4x4 QuadCab HO Cummins 6 spd. pickup, slightly BOMBed, Rancho 9000 shocks, 4 leaf helper springs, Helwig Sway bar, 19.5in. wheels w/load range H tires, Reese Titan V hitch w/28 in. extension.