Indeed, it is. In fact a laptop bag for instance is shaped in a way that makes it actually much easier to fixate than a human body.

Look at the picture. The bag cannot move vertically because it is held down by the belt. It cannot move horizontally because of the belt looped through the handles. Immobile. A human body can be held in the seat, I.e. the bum always stays in the seat. But are parts are “loose”, such as arms, legs, head. Which is why there is such a thing as brace position.

So, no, I don’t agree with those FAs. In fact, they don’t even have opinion on it, they just apply a rule without seeking to understand it - it’s the airborne version of the security staff that is convinced that he has just prevented a major terrorist attack by putting toothpaste from a transparent toiletry bag in a separate little plastic bag. Maybe he thinks that toothpaste in a separate plastic bag loses its explosive character which it had while in the toiletry bag.

Which is why, whilst I don’t agree, I don’t anrgue either.