His plan to use a VAT to pay for UBI is a regressive tax. People with lower incomes spend a higher percentage of their income on consumable goods. Putting a higher VAT or sales tax on everything to UBI puts a lot of the burden on the lower class to pay for this program. Someone with a much higher income has a number of ways to avoid this tax - they can invest in companies instead, they can buy real estate, they can take a jet to a different country and buy expensive stuff there instead of buying it in the country with a high VAT, like how people in Massachusetts buy their computers in New Hampshire.
So, he "did the math," but that doesn't automatically make it good policy.
The concept of UBI is a good idea to explore in the future. However, we shouldn't just assume that it's definitely going to work as intended, either: https://www.helsinki.fi/en/news/nordic-welfare-news/heikki-h...