that's what somebody thinks

that's what (someone) thinks

Someone is totally wrong about what they think. A: "Bob said he's planning on taking his vacation next week." B: "That's what he thinks—we need everyone here to get this project finished before the deadline!" A: "I need you to stay home this Saturday to look after your sister." B: "That's what you think! I already bought tickets to the concert on Saturday."
See also: think, what

that’s what somebody thinks

(informal) what somebody thinks will happen will not happen, especially because the speaker is going to do something to prevent it: ‘Your team doesn’t have a chance of winning.’ ‘That’s what you think!’
See also: somebody, think, what