The AR lowers are treated before they are machined, that is what the "T" number stands for in the material callout... ie. 6061-t6 or 7075-t651. That number tells you the process that the material has been through, before it is sold to machine shops. T6 means it has been solution heat treated and artificially aged. T-651, means the same except it has also been strees relieved by stretching.
These parts do not require heat treating after being machined.
Im just going off of theory here, but I believe the forging companies are telling you that they heat treat them not because it makes them stronger, but because forging the aluminum induces internal stresses. They have to heat treat it to realign the crystalline structure of the aluminum to relieve the stresses.
Also, I worked in an aerospace machine shop as a CNC machinist. The only aluminum parts we heat treated were aluminum sheet metal parts that were formed in "O" condition. These parts are completely annealed, and take a bend easily with almost no spring back.
We also made various parts for guns, and they were never sent to heat treating.
I am old school, I like the look of forged Lowers the best.
Everyone has their own preference, that is why they make different types.