Isn't UBI and negative income tax mathematically equivalent? Let's say you have $1000 UBI and 20% income tax. Then the money you end up with after UBI and taxe

Isn't UBI and negative income tax mathematically equivalent? Let's say you have $1000 UBI and 20% income tax. Then the money you end up with after UBI and taxes is 0.8x+1000 dollars. That's the same as paying 20% (signed) income tax above $5000 where you end up with x - (x-5000)*0.2 = 0.8x+1000 dollars.