Just throwing the 1000 years mark is a kinda of marketing. But the cool thing is there’s actual science behind it.

But the issue with writable optical discs is that the substrate is based on organic material. These material, usually a cyano group, oxidize over time. You can help slow that does by keeping them out of the sun, prevent heat cycles, etc. But short of storing them in nitrigen they will eventually oxidize. What’s more, CDs have their data layer completely exposed on top making the problem even more pronounced. DVDs and Blu-ray at least have a layer of plastic on top of the data layer, but that’s obviously still not 100% impermeable to oxygen.

M-Discs on the other hand use a carbon glass for the data layer. Something that doesn’t oxidize. Heat cycling night form cracks in it, so yeah I would avoid significant heat/cold cycles if you want them to last, but past that they should be really fucking stable.

It is 100 years? 500? 999? Maybe, but it’s kind of irrelevant. In optimal storage conditions (which are easily achievable) they should last many lifetimes.