Indeed it may be "marketing hype" about the level of OBAs in the Fine Art Pearl that Hahnemuhle is releasing. I don't think the OBs in Photo Rag are a threat, & I prefer that amount of optical brightening to Epson's Ultrasmooth; but my guess is that Hahnemuhle is anticipating concerns not only about the brightened version of HPR, but also the new FAP paper, which is reportedly more heavily brightened than Silver Rag & Innova Fiba.
Some optical brightening looks OK to me & is IMO worth what Hahnemuhle argues will be a return to the paper-base color. I make work-prints on EEM (heavy OB), final prints on HPR (moderate OB), & prints that I can call 'archival' on USFA (no OB). (I used to use Moab Natural until I encountered too many problems with flaking & ink crystallization.) But I regard the no-OB USFA prints as less satisfying to look at than those on HPR, because of the yellowness of the paper base. If you show USFA prints in a group show amidst other papers - especially in 'reveal' mats - the paper base has a pretty unattractive tone.
We knew that the EEM paper base would yellow in time - I believe Wilhelm said 30 to 40 years, but it seems to occur more promptly! IMO, USFA simply looks like EEM that has faded badly. That's basically what the Hahnemuhle press release is saying.
In this context, I'll continue to consider HPR, with moderate OBs, my best paper for final prints, unless the person who wants the print brings up the 'archival' issue. But I doubt the real question that Hahnemuhle is addressing concerns the the level of optical brightening in HPR.
Previews of Fine Art Pearl mention that it has noticeably more OBs than Silver Rag or Innova Fiba Glossy, & in this respect looks like EEM. IMO we'll just have to wait to see how much brightening they're defending until we can compare the new/forthcoming papers.
One more little note: Some time ago I got out old air-dried Portriga Rapid 111 & Kodak Polycontrast F prints from the 70s & 80s to compare to some inkjet paper bases, & was surprised to find that the USFA paper base is the best match to 'classic' Portriga! I know Brovira of that vintage looked brightened, but I wonder, after reading the Hahnemuhle claim that gelatin-silver paperes were typically brightened: Was Portriga ever brightened? And does it now match USFA because the OBs have faded away?