I think this is just a matter of semantics. Different stems won't change the fundamental, or inherent, stability of the bike. This is observed and demonstrated with no-hands riding. The length of the stem or, indeed, no stem at all, won't change the geometry and stability of the bike as a machine. However, different stems lengths can change dramatically how the bike feels, how the bike responds when you provide inputs to the bar or grips, and perceived stability. I think we're all taking the same thing, just using different words.