Psychopats have 10% larger striatum

Why wouldn't they? I'm not aware of any studies definitively linking brain mass to intelligence, or capacity for empathy to intelligence.

Further, I'm pretty sure the description of *paths as totally lacking empathy and emotions is not accurate.

Just lacking emotional empathy doesn't make you psychopath. You need to have other symptoms as well (like lying and manipulating people) that might require serious brain capacity. Brain could be also overcompensating for the lack of emotional empathy. Psychopaths are still able to know what people feel by using cognitive empathy. This might require additional processing power in parts of the brain.

Although the article points our rather in the direction of idea that larger striatum increases psychopaths appetite for stimulation which leads them to get in conflict with society.