Hi cubby,
This is fairly “standard” advice for all mattresses and turning it periodically or turning and flipping it (for a two sided mattress) will help the materials compress or soften more evenly and can extend the useful life of a mattress.
It’s not actually soy itself (that contains the protein that usually causes a soy food allergy) but a chemical polyol that is derived from soy oil that is used to replace a small percentage of the petrochemical polyols used to make the polyfoam. You can read a little more about so called “soy based” or “plant based” in post #2 here. I’m guessing that your soy allergy is a food allergy so the soy based chemical polyol used to make the foam inside the mattress probably wouldn’t be an issue with a soy allergy.
Phoenix