Phosphate coatings are best thought of like hard anodizing - it uses the base metal and introduced chemicals to deposit a layer that both a coating and a conversion.

It isn't a coating like paint is, and won't react to paint stripper. There are chemicals that will strip phosphate, just like there are chemicals that will strip off anodizing. Or chrome.

The porous surface of phosphate isn't just good for absorbing oil, it hangs onto paint really well and is used that way for military weapons.

It is too bad that steel bikes don't get this kind of treatment normally, but chromoly is somewhat rust resistant. The reason cars no longer rust so badly is that they now galvanize the bodies before painting (zinc plating).