Any idea why they are reporting the estimated lifespan at 290°C? Testing seems to have been done at 440°C and above.

Coz the paper gives a function for extrapolating from these tests. This is purely testing thermal decay.

10,000 years sounds like a good benchmark and isn't as obviously ridiculous as saying a million years at 260°C

It's common to perform longevity testing at higher temperatures to simulate longer lifetimes, in account of nobody has decades of time to actually perform a 1x time test.