Looks like Ghent has made some changes recently to their public transportation network and the connections from the train station to the center are not what is described in the guidebook or older forum posts. I think these changes were made effective Jan. 2024.
Google maps shows that Tram #1 (and #3) can still be used to get to the center but the closest stops are about a 7 min walk from the Korenmarkt, belfry, and cathedrals area.
It seems like the best option currently is bus P4 which does get close to Korenmarkt, with a stop just south of St. Nicholas' Cathedral.