Their post does take into account supply and demand. Giving people a big payout will leave them with a lot more money to spend on stuff, ie, the supply of money

Their post does take into account supply and demand. Giving people a big payout will leave them with a lot more money to spend on stuff, ie, the supply of money will increase massively. Unless there is a similarly massive increase in demand for money (ie, people selling new goods or services), then the relationship between supply and demand will lead to a significant devaluing of money, aka inflation.