That's the point of this. TouchID is no longer enabled. Someone unknown party approaches you, you close your lid (disabling TouchID). Then they "legally" ask you to put your finger on the sensor. You do. They didn't ask you before you close your lid.
You're thinking more along the lines that they ask you to touch the sensor and you use your fingernail razor blades to damage the sensor or something like that.
Yes, I meant to respond to other comments in here directly, but got messed up.
Others had floated the idea of locking by using an alternate finger with touchID, after the fact.