Also, the storing it outside isn't a very good test of how long it will last inside.

Also also, M-Disc is like Imax, a theater could have that label because it projects 70mm film into a dome or because it's a regular movie theater with a lower resolution than your phone screen that licenses the rights to the name. There are M-Disc DVDs that use a special archival technology that requires compatible drives, but the M-Disc Blu-Ray discs are made with regular Blu-Ray manufacturing technology. With both Imax and M-Disc, they require a minimum quality level to license the trademark, but exceeding that quality level is far from exclusive to that trademark.

My understanding is that the thing that makes M-Disc DVDs special is that they don't use organic dyes in the recording. Blu-ray discs, with the exception of the weird LTH ones, by default don't use organic dyes. Consequently, the main magic of DVD M-Disc is just the default with BDR.