Yes, but marginal rates are important, too.
If whatever fiscal policy you have makes overall nominal spending go up (or down), that would have an impact on inflation, so the central bank will remove money from circulation (or add money to circulation) to counteract.
Thus neutralising any multiplier, you might naively expect.
(Also keep in mind that when you don't tax some money, it also gets spend or invested etc.)
Which is exactly the point. What's the "marginal tax rate" on low and middle income people of the existing benefits phase outs?
The >100% marginal rates that create actual cliffs are so patently absurd that no one can look at them and find any way to justify it, but even when the combined tax+phase out rates are in the neighborhood of 80-90% of marginal income, that's still not what we want, is it?
The "universal" part of a UBI is serving the same purpose as the progressive rate structure in the income tax, i.e. you want an effective rate curve which is higher for the rich than the poor. Which means that you don't need both. What you use instead is a flat tax rate which serves as the de facto "phase out" for the UBI. Except that because it's all in one place, there are no cliffs, and there are no poor people paying higher de facto marginal rates than rich people, and there are no mountains of paperwork to apply for benefits.