My comment wasn't specific to Remington, rather a good practice to avoid rust. Heard it from duck hunters that hunt over salt water.
Never heard that 870s are prone to rust until you mentioned it.
The parkerizing on 870s makes them pretty resistant to permanent damage, but, if left in humid environments for extended periods, they can develop surface rust. Now, I'm not talking about half a century in a swamp when I say extended periods in humidity either; I mean a year under a bed in a southern home. Still, if it happens, with the rough finish they have, you just hit them with a little bit of steel wool, and they're back to new. It's a durable finish, but it can look ugly if left unattended. You could also prevent that by keeping a bit of oil on the surface while storing it or storing it in a dehumidified environment (a safe).
Well, my comment was based on parkerized models.
If you want a tough finish, you could look into Cerakote. They have certified coaters in most of the country, and the results usually come out quite well.
It's uncommon, and it shouldn't occur with proper care, which is ridiculously minimal. However, in high-humidity, low-lubrication, sustained exposure situations, minimal surface rust can present itself. Clearly, any kind of oil will prevent that, but some people are overly anxious about the possibility and want to just not care about such a shotgun. Thus, they throw on a durable finish like Cerakote.
As far as I'm concerned, though, you buy an 870 because it's the Glock of shotguns. Even if you stored it at the bottom of a salt pond for the next decade, it would probably fire, even if it weren't pretty.
Do you know why they suggest this?