They're quite upfront about it being 500T (5219) in their faq.
PE, wouldn't the archival stability be roughly the same as cross processed E6 film as, if I recall correctly, it and ECN-2 both use CD3. Or is it more complicated than that?
The remjet is pre-removed but the sample pictures don't look too bad (or obviously affected by halation). They do recommend avoiding scenes that are backlit or include strong point light sources.