Sectumsempra or not
I agree that the Marauders did not find out about this spell... It is probable that they accidently came upon it while pulling a prank on Snape, but there is no common evidence stating that they had ever used it. I think the only way they could have found out about the spell is if they actually went through Snape's school books, but James and Sirius weren't the students who studied or cared about school....
In this event, I don't think they would know, but then again it is probable.