Nematodes only attack species they are adapted to breed from, so it would be very, very unlikely that they would move from soil or inside the body of a slug to a reptile, especially as tortoises are largely herbivorous and slugs form a tiny part of their diet.
Nematodes need soil to survive, so any spilled on vegetation will quickly die.
You could ask someone who's a tortoise expert. Remember though, that if you ask an astronomer if there's a chance you could be hit by an asteroid, he/she would be bound to say "Yes."