The whole point is that paying everyone a fixed $X amount regardless of anything else is extremely easy to manage, so you can drop all the bureaucracy that builds up around welfare. But, yes, in practice it also acts as a progressive income tax of sorts even with an otherwise flat tax rate (which allows for further simplification) because delta between UBI check and taxes is going to gradually decrease as income rises and eventually becomes negative.

That said even with just personal income tax it's viable. I once crunched the numbers on what it'd take to have everyone in US receive the current federal min wage as UBI payment, assuming a flat surface tax (i.e. relying solely on that UBI check to make it progressive), and it was somewhere in the ballpark of 50%.

Of course, you can get there much easier if you go for the sacred cows such as capital gains. Raising that to the same level as regular income alone would bring a lot of tax revenue.

We could also start taxing AI, since it is (or at least positioned by those deploying it) the immediate cause why so many people are going to find themselves out of jobs.