The distinction is important, because at this stage he is only running against Hillary Clinton and whomever else decides to compete for the nomination. He is not running against Green Party candidates like Jill Stein or Cynthia McKinney. He is not running against whomever the SPUSA eventually nominates. He is not running against the radical with much better politics who is going to get swept into office on a wave of popular acclamation in a year or so.
Maybe you think that a Sanders presidency would co-opt popular support for socialism towards another four years of neoliberal hegemony. This is a good reason to oppose a Bernie Sanders presidency, but it doesn't necessarily follow that you should therefore also oppose a Sanders nomination. For instance, one could argue that a truly radical opponent would be more likely to beat Sanders than Clinton in the general election - he is, after all, a far weaker candidate. Perhaps a Sanders nomination would be a better way of bringing into sharp relief the contrast between Socialism In Name Only and a real, principled socialist agenda.
How you come down on such matters is mostly a question of strategic calculus. It does not necessarily have anything to do with supporting a Sanders presidency.