Rashi on Numbers
על כן (more lit., about this) — about this war which Sihon waged against Moab,
Sforno on Numbers
על כן יאמרו המושלים, the people who reveal the visions they have seen in their dreams and their interpretations of these visions, as did Bileam when his interpretation is introduced with the words וישא משלו (Numbers 23,7).
Rashbam on Numbers
על כן יאמרו המושלים, on account of the war against Sichon and Og they had prophesied in advance,