Theirs is tool steel; BH Spring Solutions and RDIH's position is that in the case of this particular component, quality MIM is actually a better solution in terms of strength and longevity.
FN also modified the component during the High Power production cycle, both materially and archtecturally; the final one was MIM, I believe under Leon Hubert's guidance when he was in charge of High Power production and engineering at FN; you can visibly see some of the contouring and casting/MIM differences between my replacement from Browning and my OEM 2003 production firing pin retaining plates in the side-by-side images I posted.
When you look at the Cylinder & Slide compoent, while its similar in contouring to the final FN/Browning one, there are some differences:
https://cylinder-slide.com/Item/CS0054
Best, Jon