My mom has been a flight attendant for 30+ years and has never heard of this. She tells me that if her hands are behind her back she is likely just trying to pass through a cramped aisle.
Why Cabin Crew Members Keep Their Arms Behind Their Backs When Greeting Passengers

If you’re an experienced traveler or just an attentive person, you may have noticed that cabin crew members always keep their arms folded behind their backs when greeting passengers and when walking through the aircraft cabin. When doing so, they don’t appear to check that seat belts are fastened or that passengers’ phones are turned off.
It’s really quite simple: before take-off, the cabin crew needs to count the number of passengers onboard. This takes place either as they enter or when the passengers have all sat down. The cabin crew uses a special counter to do this.

The cabin crew keep their arms behind their backs when doing the counting in order to not attract too much attention to themselves.
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Yeah this site is full of interesting bs lol.
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