The midrash is in Aramaic, not in Hebrew. IINM דידן נצח doesn't necessarily mean לשון עבר in Aramaic, but could mean לשון הווה.
I read the story as being present tense. "We're winning." The guy wanted the villagers to cheer him on.
My rosh yeshiva used to note that from the story, it seems they couldn't actually see what was going on (he says when you see the water thrashing, you'll know we're fighting). So they didn't know which side was winning, and he knew they didn't know, but he still got chizuk from hearing them cheer.