A few thoughts.

Rust is a chemical reaction, and can be chemically neutralized. While the chemicals won't replace the missing metal, they can neutralize existing rust and prevent it from re-occurring.

I kind of see two schools of thought here. One of them involves aggressive mechanical grinding/sanding methods to remove both the surface and pitting. This thins the whole panel down to the depth of the pitting. The other method involves sanding the surface rust, and using blasting, chemicals, or both to remove the rust pits. Personally, I prefer the second method - I never understood thinning the whole panel which IMO reduces the integrity and strength of the panel. So I prefer to lightly sand the surface rust away, media blast the pitting if necessary, and chemically treat the area to neutralize existing rust and prevent further corrosion.

I wonder how many times the reason that rust that comes back is mis-diagnosed as an improper repair, and not seen as "new" rust. Reason I say this is that many times I see rust repair or bodywork done, then the car sits in primer for weeks or even months before topcoat. People don't realize that many primers are porous and allow moisture to contaminate the metal under the primer. Many primers are to level imperfections and promote topcoat adhesion, but are not designed or intended for corrosion protection.