This is not a hoax.
The AD states:
This AD requires repetitive cycling of either the airplane electrical power or the power to the three flight control modules (FCMs). This AD was prompted by a report indicating that all three FCMs might simultaneously reset if continuously powered on for 22 days. We are issuing this AD to address the unsafe condition on these products.
A little investigation indicates that this isn’t the first time the FAA has had to do this. The last time they had to do something like this was in 2015-09 when they issued this AD which I obtained from here. That AD was more specific about the reason for the problem, stating
This condition is caused by a software counter internal to the GCUs that will overflow after 248 days of continuous power.
It has been widely rumored that the present AD about the 21 day action is similarly motivated, and the logic is that a timer with millisecond precision which will overflow at about 24 days.
This is all very droll, and I hope to hell that they power cycle their planes on the ground regularly and all that. My only question is this, since they are in fact running Windows under the covers, how on earth are they able to keep the thing going for 21 days?
With Windows 7 that was a piece of cake but this new Windows 10 that I have wants to reboot every night and I don’t have any say in the matter.
So whatever Boeing did to keep the damn thing going 21 days, it would be great if they shared that with the world.