It's an interesting point. How does one determine when one is past the point of being a spoiler and into being a serious candidate? It seems that there is a self-fulfilling prophesy here, wherein if one is concerned about being a spoiler, then one will begin drawing votes away from other viable candidates as soon as they enter politics, and thus spoil; but yet, in order to ever become a viable candidate in the first place, one must do that.
Perhaps the counter-argument is that you could enter politics away from an election, to "test the waters" without drawing support away from other candidates. But that's not a serious way to enter politics. Or you could enter politics at a different level, a lower level. I don't know if that's a viable way for Lessig to accomplish his aim. The same type of problems he's talking about might exist in other political spheres, but they aren't the ones that seem to concern him primarily. I can buy those arguments, but they leave me uneasy for a reason I can't characterize. It reminds me somewhat of the process of becoming the mayor of the city of London [1], where you're only allowed to run once several groups of people approve of you.
It seems like Lessig is being held to a different standard, almost, because he is entering politics with a specific policy objective in mind, rather than because he himself wants to become a leader. Most politicians enter politics simply to win and become leader, and so they can be judged by whether they win. Lessig is entering to accomplish an aim, and so it is possible to criticize his candidacy on the basis of whether he'll accomplish the aim, to a degree greater than other candidates. "By entering politics to accomplish that policy, you are being disruptive rather than helpful" is the charge, and I agree that it is a reasonable charge. A President needs to be an overall effective leader, and not ignore everything but their single pet issue. From that perspective I agree with the criticism. Yet I still feel that the charge of being a spoiler is a circular argument in some way. He's only a spoiler because he's a spoiler, and once you stop defining him as a spoiler, he becomes a serious candidate. Now of course, we have reason to believe that he is not in fact a serious candidate, which is a valid criticism, but it seems like that should be the criticism ("you are going to be a bad leader because you're only focused on one issue"), rather than "you are a spoiler".
The underlying problem seems to be with how winner-takes-all voting works.
[1] https://www.youtube.com/watch?v=z1ROpIKZe-c