MTBF of 1.5M hours is unrealistic, as the oxygen in the air will have corroded the device to an unusable state long before that much time passes, but by under provisioning the drive by 20% they certainly extend its life.
Until SSD's can sustain millions of writes before failing, it is nice to have parts with varying degrees of under provisioning to cater to the needs of a broader range of clients.
MTBF of 1.5M hours is about 170 years of constant use. Hopefully no one will spec an SSD to be in use for more than a few years. The MTBF is less of a concern as is flash wear-out (to dalethepsman's point of sustained millions of writes before failing).