That's a theoretical value, which can be estimated. That probably means, that Mikrotik has some design goals and then they choose the components accordingly. Cheaper components, lower MTBF. Most critical are the electrolytic capacitors and the cooling. (Aluminum cooling profiles are expensive to mount and use, so are the fans.) As hAPs are devices for home users, even if they die, there are normally no costs associated with expensive service staff traveling to site...
That being said, they still have probably much better life expectancy than some other cheap vendors APs.
Btw.
R (t) = e^ - (t/MTBF)
2.718^(-8760*5/100000)=0.645 in 5 years you will have roughly 65% of your hAP ac2 and ac3 s up and running
2.718^(-8760*5/200000)=0.800 in 5 years you will have roughly 80% of your hAP acs up and running
And that is for the specified temperature 25C. At 60C you will have approximately 12 times more failed units, so keep them cool!
That much for the numbers...
Woland