Interesting. I am not so certain. Scratching tends to happen when you play fight with a cat. I would expect people who could be described as depressed would care less about being scratched by a cat than those that aren't and are therefore more likely to be scratched.

Also the conclusion of the study you linked is that toxoplasmosis isn't linked to depression but scratching is. I found this unexpected given the wording of your post.

Edit: also when you clip a cat's claws it can't really scratch you. I'd also assume someone who can be described as depressed is less likely to clip or get their cats claws clipped often enough to prevent scratches.