Where did you get the idea that rotating is "to keep it from setting" and not mixing?

https://en.wikipedia.org/wiki/Concrete_mixer#Concrete_mixing...

"Special concrete transport trucks (in-transit mixers) are made to mix concrete and transport it to the construction site. They can be charged with dry materials and water, with the mixing occurring during transport. They can also be loaded from a "central mix" plant; with this process the material has already been mixed prior to loading. The concrete mixing transport truck maintains the material's liquid state through agitation, or turning of the drum, until delivery. "

Y'all both right.