The physics seem sound to me but I'm sure it's not that simple?
It is that simple - otherwise I am sure the RAF would not give that advice. Of course there is no shortage of speculative "but what if's?" but the lower the energy at impact the safer you should be. Please forgive the "war story" but interestingly, in another era where yours truly went from flying Mustangs (tail wheel type) to Vampires (tricycle type), wheels up policy for forced landings on both types of landing gear on rough surfaces, was still in vogue; mainly from our ignorance and lack of any official advice to the contrary.
Later when I sort out how to put the photo on Pprune, I will post a classic photo taken in the 1950's of an RAAF single seat Vampire on its belly where the pilot decided to force land wheels up in a field after his formation leader got them lost them lost near Cooma NSW on a navex. He was uninjured in this case. (Gooday, Bill, if you are still around!)
It wasn't until US and RAF military flight safety magazines began to filter down to RAAF squadrons, it dawned upon people that wheels down forced landings into unprepared surfaces for new fighters like the Sabre and the Century Series fighters were proven to minimise back injuries if the wheels were down. Might also be safer to eject on those hot ships, too