Belgic Confession article 18 says in part:
"Therefore we confess, against the heresy of the Anabaptists who deny that Christ assumed human flesh from his mother, that he "shared the very flesh and blood of children"...
Doing some research, it seems Menno Simons affirmed the humanity of Jesus, but thought that taking His flesh from Mary would impute sin to Him. This false inference led him to think God must have prepared a body for him without becoming /our/ flesh. That would be sinful flesh, he thought. This is the old gnostic error, that our sin is inherent to the physical flesh. Following it, God must have created a new "kind" of flesh body for Jesus. This ends up rejecting that He was made like us in every way, except for sin, I think.
This is why the Belgic confession is so adamant to reject his error. It can be easy to dismiss him as a heretic, denying the humanity of Jesus. But I don't think he goes that far. He thinks God accomplished the incarnation in another way, than by Jesus taking the human flesh of His mother. Peculiar and wrong-headed, yes. Leads to a denial or diminishing of the physical world God came to save, yes. But be careful branding people heretics too quickly. This seems a decent summary of Simons.