Oh no he's not!

According to Equipe on the 27th of Jan the president of Racing Metro, Jacky Lorenzetti, says that Sexton is not going to be playing with his side next season.

http://www.lequipe.fr/Rugby/Actualites/Sexton-fait-le-buzz/345809

He says they met with Sexton but did not agree to sign him, especially not for that much money (€750,000). He says that the rumours that Sexton signed for Racing are lies. Then below it suggests that one of Sextons relatives said that he told them he will be playing in the top 14 next season in either blue, yellow or red. They author speculates that he could chased by Clermont-Ferrand, Toulouse and Toulon. They suggest that Racing could try to bid for him for €500k. Strangely then they list Racings signings and think that Racing is at the end of it's recruitment drive.

Yes he is they were just worried that they might get a fine for saying they signed a player early. You can't announce signings until march or april.