Absolutely true.
How does his sinless human flesh derive from sinful human flesh?
Because he is God.
We're not talking about "corrupting God". We're talking about the formation of a being, both Divine and human, from the flesh of another human.
You are implying Christ would be corrupted if he were born of sinful human flesh. Although fully human, Christ is unlike any other human because he was born without sin. Our savior is the only human who can claim perfection.
We're dancing here.
"Because he is God" does not mean that anything goes. God is light and light has nothing in common with darkness. There can be no evil where God dwells for God is all holy. To the extent that God is present, evil is not and vice versa. Think of it in the same way as one of the laws of physics.
Just "because he is God" does not mean that God can suspend His own Divine, holy nature to partake and receive sustenance from that which has been tainted by sin. To suggest otherwise is to infer that God's nature is not "all holy", which I think we're both agreed, is untrue. To do so would be to go against the fundamental nature of God.
People use the term "because he's God" unthinkingly to imply that God can do whatever. It's false. God does not sin, for example. Not only that, sin cannot exist in the presence of God. The two are incompatible.
That is why the womb which bore Jesus must have been sinless.