How could they see chemicals? That's silly.
I have been using Miracle grow potting soil for years and years! I have never found anything like what you are describing.
Are you sure that it is not bags of top soil or Garden soil that they were using?
And even if they are owned ultimately by the company that is responsible for Round Up, that is quite a leap of faith to think that there might be an issue there.
40 years of growing vegetables, flowering vines, tropical plants, Dahlias and over twenty different types of flowers tells me a different story.
Reviews huh? Don't believe them. My eyes tell me otherwise.