While Richard knows more chemistry than I do there is at least one gaping hole in this argument, which is "stainless steel". What is "stainless steel"?

While I cannot answer that too expertly I can tell you this:
Stainless Steel is not a single alloy, but rather a whole CLASS of alloys all of which have different properties. S/S can be customized for the application needed.

For example, many knife blades are made of stainless steel. Much work has been done to create an sub-class of alloys that is nearly as fine-edged as carbon steel, but it still stainless. These tend to be harder for their edge-holding abilities, and their strength when abused. However, they are fairly easily damaged by chemicals, and do NOT do well when left wet--if dried they won't rust, but wet they can, easily. I have several that have water or chemical damage. Some ARE tough enough for both a good edge and chemical impervience. They are expensive--surgical steel in scalpels is a good example.

Other stainless steels are softer, better looking, and stand up to the elements better than knife grade--but they aren't for knive blades. These S/S will generally be impervious but if totally immersed will, over time, rust.

Then there are the chemical-grade S/S. I don't know a lot about them but I do know they are designed to remained immersed without rusting or even pitting. Some are even used for stents in the body.

So when stainless steel fittings are going to be used for a pool, you'll need to know what kind of S/S it is, and whether or not it's rated for total immersion. If it's not, it WILL rust--and too quickly to!