That's a single data point presented by someone who has a commercial interest in drumming up cash to promote their idea of 'archival' to a sociologically specific marketplace. And I recall that the original claimant that story came from tried to claim it was only 'indirect' daylight they were exposed to - and they didn't consider any other environmental aspects of chemical exposure etc. Pre-1970's most photo paper manufacturers weren't interested in the concerns of the 'fine-art' marketplace to any great extent - and the warmer toned papers used for portraiture etc (ie the prints average people were perceived to want to be long lasting) generally had nil to low brightener & often had a toning step that sulphided the paper. You have to decide what is perceptually important to you & whether you are going to be exposing your prints to intense UV containing light for hours per day, 365 days of the year for multiple decades. An awful lot of papers (be they printmaking or photographic), no matter their additives or not, will change in that sort of environment. And the nature of environmental reaction with the OBA's cannot be discounted - dyes can have bad reactions with 'archival' buffering in mount materials, just as siderotypes can. Whether or not storage/ display materials pass the Photographic Activity Test is a more likely indicator of future problems than someone making claims on the basis of baking prints for a few weeks.
The other thing is that top layer of photo paper coating packages nowadays generally includes some degree of UV filtration - and the Ilford paper(s) with washable brightener is Multigrade Warmtone FB and (I think) Art 300 which may have either the same system, or less brightener. Foma's warmtone papers may or may not have some, but they also have dyes to adjust their base colour - their cooler papers are likely to contain them.
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