As far as I know, the only foods suitable to use on the Medifast plan are the Medifast foods. They are specifically formulated for the Medifast plan. Nothing else is going to be the same, and no one at Medifast would tell you it's OK to use other brands.
That said, you could try substituting if you wanted to. Read the nutrition labels and see what the differences are.
I think it's too bad that the seller didn't say in advance that they were selling Wonderslim in the package.
"My religion is kindness." --His Holiness the Dalai Lama