Interesting to see that the passenger in the video contradicts that explanation with his testimony of the pilot saying the instruments went blank for a few seconds.
The pilot might not have given the true reason to passengers.
I find the dark screen report persuasive because there's an air worthiness directive against that 787 which will cause the reset of flight control l computers. That would be consistent with both the dark screens story and the loss of control, if true.
Except that the expected loss of control would not have suddenly changed the direction of the plane, but instead kept the direction from being suddenly changed.
Now there's word from Boeing that seat switch can be inadvertantly tripped by touching the cover.
Would they actually lie though? That's not very professional.
AFAIK, the primary objective of pilots when talking to passengers in situations like this is not to be 100% honest, but to avoid a panic. In this particular instance, he might also have tried to protect the flight attendant or avoid personal/company embarrassment. Although as a passenger, "I was accidentally pushed against the controls" would comfort me much, much more than "the controls of the entire aircraft suddenly went black and we have no idea why".
Guess the real reason may also provoke anger in some passengers. You can argue with attendants if you know that crew did it, you can't really argue with a plane or forces of nature. For those passengers providing fake reason would ensure a safe flight.
Because "oops, we almost just accidentally killed all of you" isn't a nice thing to hear mid-flight.