This doesn't directly concern me or my family because nobody who owns a home is moving for the foreseeable future, but the math sounds shaky:
One, the gain on which you are taxed (all exclusions, exemption, credits, etc. aside) is the sales price minus the basis, so if you bought it for 90K and you sell for 100K, normally only the 10K net gain would be what you would be taxed on, and;
Two, it mentions an amount above which it applies and then does the math as if there is no such floor.
I'm not in favor of the tax, but I don't like getting drawn into supporting flawed claims, either.