"Law of supply and demand" is a thing, but what you are calling "supply" is not supply. It is production.There is no law of production and demand. You can twist meanings by calling it "new supply" or "supply of new coins", but in the end it is not supply.
The number of bitcoins (also called "supply", unfortunately) is constantly increasing toward 21 million, and the halvings slow down the increase. As the number of bitcoins increases, the bitcoins available for sale, the "supply", also increases. Since the number of bitcoins is always increasing, so is the supply.
Regardless what you could call it, the main point is that the production per block is cut in half. While the supply is still going to increase as it should, it's going to increase significantly slower after the halving hence I think using the "supply and demand" argument is valid in this case. But yea, we won't feel the effects immediately. It's going to take a good number of months.