It does work, but their definition of permanent is just ad speak.
Moisture will continue to encourage any remaining rust & it can only last so long. Rust will swell up & blisters will reappear. It all depends on usage, storage etc.
A dry location like Arizona it will last a lot longer. Or rarely driven stored in a/c would help, of course.
Fillers can be added to Epoxy Resins' like Gougeon Brothers West System that absorb much less moisture. You have to mix in 'Fillers' to make a bondo type material. They are marine products.
Many fillers can be used, for various properties and some such as Silica, are very hard to sand. Microballoons(plastic) being a much easier to sand soft filler, still can absorb moisture. Glass beads a little harder & less absorption.
Graphite can be added for a slick surface, such as racing boat bottoms. Athletic field marker, a powdered Limestone absorbs no water, after curing, but is very hard & not hand sandable.
I'd think newer fillers are available that I don't know about. Different hardeners are available for different temperatures, and a 'Slow' hardener for higher temp gives a longer working(green) time. Working(filing, etc) filler while green is necessary, like too much hardener in poly bondo.
Bottom line it would last a while longer, but is more expensive & there is a learning curve.
Sun exposure will degrade the resins, is why Polyesters are always used for most fiberglass. Sealing(epoxy paint)can prevent moisture absorption, but backside can't be sealed & will be absorbed more anyway.
Sealing rust after Ospho(rust converter) does help retard degradation.
No fillers are permanent.