In the sense that “poverty exists not because we cannot feed the poor, but because we cannot satisfy the rich,” you’re correct that putting a floor on wealth can’t solve the whole problem by itself. But that doesn’t mean it can’t help, especially when you consider:

1. UBI’s ability to lower the opportunity costs for things like retraining and entrepreneurship by helping people sustain themselves during that gap in earning, which gives them power to get themselves out of exploitative work situations. Addressing that power inequality between employers and workers is real, and is something UBI accomplishes a lot better than schemes, especially stuff like welfare with work requirements that do the exact opposite by forcing people to take any job, no matter how shitty, to remain eligible.
2. That the money for it has to come from somewhere, and most proposals for funding UBI involve making other taxes more progressive.