There is no way, at this point, that Bernie Sanders can win the nomination. The primary in DC is not relevant to the election at large. Bernie is only staying in at this point to continue to bring attention to his liberal platform. For instance, he is raising the idea of DC statehood. There is no way for him to win, but he can still influence.
The race for the Democratic Party's presidential nomination is effectively over. Hillary Clinton has secured enough delegates (pledged and super delegates) to secure her party's nomination. Short of an indictment of Clinton, Bernie Sanders has no chance of winning the nomination. In short, it is very unlikely that the DC primary is still relevant.
The race was over before the DC primary. Both sides have had their nomination in place for some time now. I am unsure what Sanders thought he could possibly accomplish by continuing on with his campaign, but he should have bowed out a long time ago to give Clinton time to unite the Democratic party.
By numbers alone, the D.C. Democratic primary isn't exactly important. But it could give Sanders a last-minute confidence boost or, on the flip side, provide another convincing reason that Clinton should be the party's nominee, thus encouraging party unity as the Democrats head into the general election against Trump. At this point, that would be an ideal outcome for Clinton, who desperately needs to woo the hardcore Bernie Bros over to her side.