The rust-proofing process, they drill their own holes and plug but agree may use some existing. I’d be concerned about developed rust that will be covered over? Inside frame and all hollows. Right at jacking point is a drain hole, those should be maintained open, others in rails also.
All the vehicles I have owned for the last 20 years or so, I have had sprayed with Krown or Rust Check on an annual basis. IMO it has made a big difference. No body panels rusting out from the inside on any of them. When I eventually decide to sell them, I get top dollar.
My other current vehicle is a 2008 F350, that looks almost as good the day it rolled out of the factory. My neighbours 2011 F250 that has never been rustproofed, and has about half the mileage mine does, looks like a much older truck, due to the rear fenders starting to rot out. That's proof enough for me.