A lot of Anabaptists refuse to call Mary "Mother of God." We have had several threads in this forum discussing this. From a Roman Catholic perspective, refusing to call Mary "Mother of God" means denying Christ's divinity or Christ's humanity, or makes them a Nestorian, separating the two natures of Christ in some way. I, personally, as an Anabaptist, have no problem with the title "Mother of God," understanding that it means Mary gave birth to Jesus, who was, from conception, fully human and fully divine.
I don't know where Fr. Carol got the idea that Anabaptists hold that Christ was not truly human. I have never heard this before. I think he is mistaken. As far as I know the only place he could have gotten that idea is from the refusal of some Anabaptists to call Mary the mother of God.
Here is a pretty good
article on the Christology of Menno Simons, the most prolific and thorough early Anabaptist theologian. It supports with more detail what Freeinchrist posted. As you can see, Simons taught the full humanity and divinity of Christ. His view of how this came to be, though, was somewhat unorthodox, and not necessarily followed by all Anabaptists, but still believed by some here. He believed that Mary gestated Jesus, but gave nothing to his human flesh; rather Jesus' human flesh was a special creation of God in her womb.
As far as I know, there is no evidence to sustain Fr. Carol's assertion that Anabaptists do not believe in the humanity of Christ. The only way this claim can be supported is to define "human" such that it excludes a creation of human flesh separately from the creation of Adam. I think Anselm's
Cur Deus Homo does in fact assume that the definition of humanity excludes a separate human creation. However, if Menno were forced to accept either that Christ received his humanity from Mary or that Christ was not fully human, he probably would choose the former, although he actually believed this to be a false dichotomy.