It doesn’t matter whether these amounts are the same or different, as long as replacing the return fuel with the new device can lower the spacecraft’s weight enough. So, this is irrelevant.

It’s irrelevant whether the Mars-manufactured fuel would be exactly the same as the Earth-manufactured fuel, as long as both of them can propel the spacecraft.

The argument only asserts that it’s now possible to send a human journey to Mars and back, without any claims about how much living space the crew would have. That makes this irrelevant.

The argument is focused on the physical possibility of a human journey to Mars and back. The amount is would cost is irrelevant.

If we negate this—if the new device weighs the same as would the return fuel, or more—then the argument falls apart. There would be no reduction in weight, so it wouldn’t be possible for the spacecraft to launch at all. That’s why this is necessary.