Wouldn’t martians still be subject to Sheva Mitzvot B’nai Noach??
Yes. And therefore the LR’s argument is that they would need to know about the Torah, and since they can’t know about it therefore they must not exist. My question is that until recently the same was true for the vast majority of humanity on this planet, who undoubtedly existed, and were obliged to obey the 7MBN, and yet had no way to find out about them.
(Remember that it is not sufficient for a BN to keep the 7 mitzvos because he figured them out himself and they make sense to him. He must keep them because Hashem commanded them to Moshe Rabbenu, in the Torah. And for this he must have heard of Moshe and of the Torah.