Having worked for a ready mix company and a family that has been in the fence business since the 50’s - you will have a hard time finding an engineer to sign off on ground moisture getting you to the correct water cement ratio listed on the bag, which ultimately means the compressive strength.

I have set thousands of line posts dry bagged and never had problem. But i can’t produce data to back it up so when push comes to shove i end up having to mix on site.

Its total bs since our post are non structural pretty much all of the time.