The argument concerns the existence of sentient beings on planets OUTSIDE our solar system. Whether there are sentient beings other than humans in OUR solar system is irrelevant.

We’re trying to prove that determining the existence of sentient being in other solar systems requires that they be at least as intelligent as us. (B) establishes that IF there are beings at least as intelligent as us, they would want to communicate with other sentient beings. But this doesn’t prove anything about whether they NEED to be at least as intelligent as us in order for us to determine they exist.

We already know from a premise that we can’t send a spacecraft to planets outside our solar system. So it doesn’t help the argument to establish some condition that would prove we can’t send a spacecraft to a planet; we already know this. Also, we’re trying to prove that a sentient being in another solar system NEEDS to be as intelligent as us to determine that they exist; (C) establishes what happens IF there’s a sentient being as intelligent as us. That doesn’t prove that they NEED to be as intelligent as us.

(D) establishes that the only two options for determining the existence of a sentient being are sending a spacecraft, or communicating with it. We know we can’t send a spacecraft, so that establishes we have to be able to communicate with it. And we know from a premise that communication requires that the sentient being be as intelligent as us. That’s why in order to determine the existence of those beings, they must be as intelligent as us.

We’re trying to prove that a sentient being in another solar system NEEDS to be as intelligent as us to determine that they exist; (E) establishes what happens IF there’s a sentient being as intelligent as us. That doesn’t prove that they NEED to be as intelligent as us.