It's very true I'm afraid. Left wing people say it all the time "Building more housing won't reduce the price". Just look at this thread for numerous examples. It's widespread in the real world too, (to a lesser extent).
I don't know how these people explain the world:
Why would the price of gold fluctuate, if not for demand and supply?
Why would drugs be more expensive in prison than outside, if not for demand and supply?
Why are aeroplane tickets less expensive off peak, if not for demand and supply?
Why does Uber have surge pricing, if not for demand and supply?
It's such a central concept to how the world works, I don't know how people can get by without it.
If they had a house to sell, and no one was interested, would they raise the price to try and get a buyer? Would they lower the price if there were lots of people interested?