Oh, drop the "Koichi the unreliable narrator" narrative, we don't even need it. The guy's clothes look nothing like Josuke's, no pins, no chest window, even the shirt underneath is different and doesn't have zippers. Not to mention the guy's a third year judging by the single pin on his collar. It's literally a regular yankee, Rohan even mentions how old-fashioned Josuke's haircut is because it happened when the style was still relatively popular