I would still say that threatening someone with turning them in would be +Hard. “Do this or else something I’ll make something bad happen to you” has the finesse of a sledgehammer.
The Traits are more like Approaches in Fate, as opposed to abilities in D&D. Hard can be social, emotional, or physical. You can even be a +Hard hacker, who uses brute force tactics to break through firewalls and password protections.
ETA: It's also totally fine to play a little loose with the Traits. If someone has a good argument for why they want to use +Smooth instead of +Sharp for a particular action, I say let them. It won't hurt anybody.