> The disc itself does have a printable white top, but also features branding in the centre consisting of areas of clear hub. The underside looks pretty much the same as a normal BD-R, being metallic chocolate brown in colour. It is visually indistinguishable.
...
> This bought me to a realization – unlike DVD’s, almost all regular BD-R discs do not use organic dye recording layers. Only BD-R LTH type discs do, and they have been shown to have poor longevity. With one of the main reasons for causing disc failure no longer an issue, it seems possible that this disc is not materially special compared to a regular BD-R.
The review also mentions that the text on the disc packaging quotes a lifetime of several hundred years, so that hasn't changed either. The actual M-Disc testing certification mentioned in the review quotes a 95% CI expected lifetime of 530 years and a mean expected lifetime of 1,000 years (meaning only half the discs can be expected to last that long).
M-Disc isn't an open standard, so the criteria for what is and isn't an M-Disc aren't publicly known. I think it's entirely possible that standard inorganic BD-Rs pass whatever criteria are necessary to call something an "M-Disc", and that there's never been any real difference between a high quality BD-R and an M-Disc BD-R.