As far as I understand it MTTF measured in device hours is meant to give expectation on how often to expect a failure when you buy a pool of devices and run the

As far as I understand it MTTF measured in device hours is meant to give expectation on how often to expect a failure when you buy a pool of devices and run them a reasonable amount of time (I.e. a number to tell you when you buy 2,000 of this how to calculate your expected failure interval) not necessarily "how long an individual device is expected to keep working".