Double linear actuator boom movement
Is the reasoning wrong?


Most probably it is not wrong, but its effect is small enough to be negligible (at least compared to other factors, like ensuring an error-free building experience). In serious engineering applications this effect would certainly have to be taken into account.


  why are the Lego designers seemingly not aware of this and haven't they addressed this in later technic sets?


I took a look at my copy of the Bucket Wheel Excavator, and found that there is a minuscule difference in the angle of the double linear actuators between the highest and lowest boom elevation configurations:





I don't have the relevant values right now, but I'd guess this angle of rotation does not result in a measurable difference in the lenghts of the two actuators. Or if it does, it is certainly absorbed by the flexibility of the structure without causing any harmful amounts of stress.

I think the same applies for the other such sets as well, but I'd welcome anyone providing data to either confirm or disprove this theory.