If there is literally no difference then what benefit is to be gained from the added complexity?
One of the big advantages of a universal basic income is that it gives you effective progressive taxation without a complicated system amoral people can game. If you have different tax rates then it creates opportunities for tax arbitrage and rich people can avoid paying the higher rates by doing things like paying a middle income salary to their otherwise zero-income college-attending children who pay a lower rate.
But flat tax + UBI is inherently progressive, so that's it. No phase outs + no tax brackets does exactly what everyone wanted anyway, but everyone universally pays the same rate and receives the same UBI so there is nothing to game. You get one choice, what the flat tax rate should be, which sets the amount of the UBI and determines how progressive the system is. It takes away all the knobs that politicians who don't know what they're doing (or corrupt politicians who know exactly what they're doing) can use to screw up the economy or shift more of the tax burden from the rich to the middle class.
A universal basic income has rich people paying poor people and middle income people paying themselves. A basic income with a phase out instead has middle income people paying poor people so the rich can have a tax cut.
This does not have to be true. You can simply phase it out and then adjust the tax structure accordingly. It is a zero sum game, you lower taxes by the same amount you cut the basic income for that person. At the high end you take away 11k basic income and reduce taxes by 11k, somewhere in the middle you reduce the basic income and taxes by 6k each and at the lower end you do nothing.
With matching tax structures there is really no difference between paying an universal basic income to everyone or only paying it towards the lower income range. I am not disagreeing that a flat basic income might be a lot simpler than trying to carefully adjust tax rates.