The argument confuses natural death with intentional killing. Many embryos may be lost spontaneously, but that does not make deliberately killing one morally equivalent to a miscarriage; just as the fact that many infants die naturally does not justify killing infants. The extent of spontaneous embryo loss is itself uncertain, and vulnerability does not make human life expendable.
The relevant question is not whether the child is viable, wanted, or likely to survive, but what the act intentionally does. An embryo is a living human being from conception, and abortion deliberately ends that human life.
“Murder” is commonly a legal term with jurisdiction-specific definitions, so “intentional killing of an innocent human being” may be more precise. But morally, that is exactly how Catholic teaching characterizes direct abortion: the deliberate killing of an innocent human being in utero, regardless of age or stage of development.
This is how Tolkien would have seen it, this is how billions of human see it - because it is true.