Yeah you are assuming so many things there. Supply and demand definitely works but when you simplify out every detail you come to some bad conclusions. Giving people money will cause only specific things to raise in price. Things like houses where the supply is inelastic. Prices will rise because more people can buy by them but there are virtually only X amount. Things like food however will stay pretty steady because if there are more people competition will cause merchants to bring more food and undersell the competition.

In this examples test phase they also aren't sending everyone money. They are randomly seeding money to several households in the community, which will also influence its spending. Because this is surprise money it probably has a higher change of going into deferred payment. I am not convinced this test a good way to test something like a UBI but its not simple as lesson one. I think its a huge mistake to think you've learned everything and can holistically judge broad scale policy from just lesson one but thats just me.