Answer

He probably means the rule of “seizing something on behalf of a creditor when that harms others.” But there the discussion is about acquiring something (a found object or debt collection) for one person at the expense of others. And even there, the medieval authorities (Rishonim) disagree whether the rule applies when the beneficiary appointed him as an agent, or whether it was said only regarding the rule of acquisition on another’s behalf. Also, if he can acquire it for himself, he can acquire it for someone else as well (though this requires further analysis here—whether this is considered a case where he could acquire it for himself, since he has no car here, and one could debate that). But here, the person holding the spot is not acquiring the place for the one for whom he is saving it; he is only preventing others from entering. In my opinion, this is not really a matter of agency or acquisition at all. In this case, it seems that everything depends on the accepted custom. I think the custom is that it is permitted.