Yep and no
Traditional fibreglass construction has wax in the top flowcoat so epoxy may not stick well.
So sand the edges, clean it with acetone very well and yes it will be fine. But permanent stick - maybe not.
It is so easy to lay a surfboard flat and dribble in fibreglass resin, so I dunno why you would repair a surfboard with Knead-it.
Knead-it is for things that are a bit harder to repair and you just wanna hit the water, eg: a windsurf or SUP and you are on holidays 100km from a town that has running water
Else, do it right.