The article didn’t say that. This is what I read in the article:
“The telephone poll, conducted Sept. 14-16, finds that 51% of Louisiana voters prefer Mr. Cassidy as their next senator, versus 38% who favor the incumbent.”
This is a FoxNews poll that shows two scenarios: one head-to -head and the other with Maness in the mix. Look at Realclearpolitics.com’s breakdown of the race.