Yes, this rule exists. When you enter the country, you can check the availability of this amount or the availability of credit cards. Usually they check those tourists who cause suspicion of the immigration police, that is, they are checked selectively.

They do not demand, but they can ask to show. Since in most hotels in Thailand, as a rule, only breakfasts are included in the cost of accommodation, it is considered that when arriving in Thailand, the tourist should have enough money to allow him to enjoy the rest.