The blog post says "When the car is approaching an overhead highway road sign positioned on a rise in the road or a bridge where the road dips underneath, this often looks like a collision course. [...] If several cars drive safely past a given radar object, whether Autopilot is turned on or off, then that object is added to the geocoded whitelist."

So as i read it, the system does think it will collide but ignore it if the whitelist says the object is safe.

The paragraph after that says basically once they have enough data, the system will start braking on unknown objects with gradually increasing force as the confidence level rises. So basically, it will brake on unknown traffic signs but only slightly, as the confidence level shouldn't get too high.