Or HaChaim on Numbers 16:11/אור החיים על במדבר ט״ז:י״א

1 לכן אתה וכל עדתך, "Therefore, you and your whole congregation, etc." The word לכן is equivalent to an oath. Moses swears that Korach and associates are gathered against G'd (not against Moses). Moses did so in the hope that when they realised this they might yet become afraid to continue this quarrel and not go through with the test to determine who was fit to be a priest.