Ahh, i see you're new to road design. I too are new to road design and has been ignorant for my whole life, so lemme walk you through what i've learn for the past few months:

This is not traffic calming, this is a bunch of yahoos putting obstacles in the road and congratulating themselves.

Nope, it is traffic calming. It use a bunch of tactic to make driver uncomfortable so they slow down naturally without resort to active enforcement, including but not exclusive to narrowing down the street, use road bump or raised crossing, and make the road ~~gay~~ not straight.

And wtf do they think will be improved by blocking the turn lane?

It's a tactic known as road diet. On that particular street from the footage, the turn lane(or median) combined with the two left and right lane are making the road too wide, this sort of encourage driver to drive fast because they feel safe to do so, making the residential road unsafe. The quickest and cheapest fix is to raise the median, eliminating the unnecessary turn lane which in turn narrowing down the street, so they plop down some dirt-filled tire in the middle lane. It also create a pedestrian island so whoever crossing the street will only need to look at one side at a time to cross, and also narrow down the street making it easier and quicker to cross. Of course the proper way to do it is to eliminate the median and narrow it down from the two side, either making the pedestrian path bigger or adding protected bike lane, but guerilla tactic often need to be quick and effective, and this is quick and effective.

A bigger question is why these MFers keep calling the turn lane a median?

It's a road engineering term. In some place, median tend to be a reserved space that separate two traffic, usually left unpaved, or raised to properly separate traffic, decreasing traffic conflict thus decreasing accident, while also create a safe space for pedestrian. In North America it's used as a shared turn lane.

And they seriously think it’s good that their obstacles flipped a car? wtf with that? They should be arrested for endangering or sued for the property damages.

I don't think they feel good about it, but the driver also should feel bad about their own bad driving behaviour. If a car can hit an obstacle placed at the median and land on its side, then two thing is true: 1) they drive too fast; 2) they drive distracted. Drivers need to know they need to be in control of something weight at least 2ton pound, the consequence of it hitting someone is heavy. Replacing the tire with kids, and the story will be in different tone.

but how about something that improves safety rather than risks danger

I think safety should be applied to both driver, motorcyclist, cyclist, and pedestrian, but often time when people think about safety they almost always only think about the drivers, so they make the road wider and straight, while it slowly eats away pedestrian's right to safety. It's the sheer ignorance and lack of care toward anyone who isn't in a car that rile up these people and make them take action.

It's understandable that driver want to drive on a road without much resistance, the urge of unleashing that power is understandable, but at the same time, pedestrian doesn't want to die too.