I have seen flash rust over parts of machinery when that part was stored for some time. Not an issue and that is from a guy who restores old tractors and paints them and does a magnificent job. He can't pre coat with anything as that messes with the primer. So he lets it flash rust, cleans it off when he is ready to paint it before he puts it on. Perfectly smooth surface, no pits, no penetrating corrosion.
Rust is actually the main protection system on some structures. Once it rusts to X degree, no more corrosion. Yes it is lethal in other circumstances per re bar in concrete.
A blanket statement like that does not do anyone any favors information wise.
First of all its not the entire firearm, its discreet parts. As long as there is no pitting, there is no damage.
And yea, I have personal experience with that. 60 years ago, my dad died in a boat wreck. Hunting trip. They found the boat and his gun was still stored in the cabin, they flushed it and sent it to my mom.
She in turn turned it over to a gunsmith we had know since we were wee lads, he went through it again, flush, cleaned, all good. We hunted with that rifle for 40 some years. Then one day my younger brother took it down to confirm sight in, first shell that came out was a wrinkled mess. He stopped and consulted his much more gun savy brothers (two of us were deep into guns).
Ok, no, don't shoot it ever again. Best guess is corrosion got into the metal and its gone from no issue to its dangerous in a year. It never was abused water wise, always cleaned, seldom hunted if ever in the rain, alwyas cleaned.
So yea, it can happen, worst set of conditions, it was in salt water for a week and unknown how much time passed before they got the boat and then the gun to someone to flush it (exposed to oxygen would be the time it started to do the bad stuff)
We replaced the barrel on it eventually and still shoot it.
So no, a bit of controlled flash rust is not an issue.
Compressed air has a lot of water vapor in it. A hidden passage can trap water and you have rust occurring. As its hidden and continues, its the kind of rust that will damage things though in a mechanism, not likely a serious problem though the frame could suffer.
Heating dries all water and the vapor leaves