I do not have a VP9, so take what I'm saying with a grain of salt:
Most of the common stainless steels are not magnetic either. If I had to guess, it's some kind of austenitic stainless steel, not titanium. Steel is much cheaper to manufacture, and is easily sufficient for an ejector/trigger housing assembly. Titanium is softer, and doesn't tend to hold up to repeated impacts or abrasion as well as steel.