You don’t understand the argument.
First, what does it matter if “someone” refers to a person? Even if that’s true, how does that insight help you challenge the argument? You still have to address Marquis’ point that what makes killing an adult wrong is the same thing that makes killing a fetus wrong.
You ask: “Why would you apply the reasoning that something’s worth is based on future value and apply it exclusively to humans?”
We already do this. You’re familiar with the criminal justice system, are you not? We tend to think killing an innocent human being is a crime that deserves a great punishment, whereas we don’t imprison folks for mowing the lawn. That means we already accept, and build into our legal code, the notion that killing a human being is supremely and uniquely wrong. Killing plantlife, on the other hand, is not a punishable action.
What Marquis is saying is that the wrong-making feature of killing an innocent adult — which we all recognize as being really, really wrong — is no different than killing a fetus.