I agree it looks too small to be a concrete block. But back to basics, the C172 wing will produce enough lift, at a certain angle of attack, to lift circa 1100 kgs off the ground at around 60 knots. Tying a couple of hundred kgs of concrete to it will not do anything if the wind is strong enough.
It just gives you an increased level of protection on the usual stormy days in the UK where winds gust to 30 or 40.