UrRong, indeed! It is true that an effective altruist could become President of the United States—it's definitely not just a laughing matter! As Nikhil pointed out, our current president is proof that an outsider can win the election, and an effective altruist does provide a refreshing alternative to the status quo.
But let's not forget the context of the question. Nikhil initially asked Peter Singer if he thought an effective altruist could ever become the President of America. Singer laughed, implying that it was highly improbable, and he is right. The fact is that the current political system favors candidates with large amounts of money and influence. A true effective altruist, who may lack those resources, is likely to have a hard time breaking through the noise.
Of course, it is encouraging to see effective altruists in public office and to know that they can make a difference. But to suggest that they can win a Presidential election is still a bit far-fetched. To put it in Peter Singer's own terms: with who we have in office now, it's safe to say an effective altruist would have an uphill battle getting elected.
So while an effective altruist could theoretically become president of the United States, it may be wiser for Nikhil to focus his energies on more achievable goals—like running for mayor or city council! UrRong, indeed—but let's also be realistic.