I suppose that is true, although in the case of M-Disc DVDs, they did test the whole assembly and proved that it was outlasting their competitors. I suppose my bigger issue has to do with the statistics around accelerated age testing and how relevant that might be. Polycarbonate is probably quite stable at the intended archival conditions, but the adhesive and reflective layer of the BD-R may be just as vulnerable as before in regards to moisture and oxidation.
Regardless, I think the even bigger issue is obsolescence - we're seeing less and less optical drives being shipped with computers, and the media is falling out of favour with most users. Even if the discs last 20 years, I highly doubt that reading the media would be easy as drives will be rare, and interfaces may have changed by then. As a result, I suspect they're selling these discs with the knowledge that it may not be relevant in the near future (and nobody would know even if it failed before they said it would). Also, it seems peculiar to me that none of these discs are sold with any warranty - occasionally, lesser commodity discs are supplied with a limited warranty of disc replacement for defects but these have no such statement at all.