UBI is absolutely a kind of socialism; it may embrace the same kind of 'capitalism' we have now, only in that what we have now and sometimes call 'capitalism' is a system which blends many elements of socialism with some structures of capitalism in an attempt to address the kind of problems with capitalism that were the center of the critique by the socialists who named "capitalism" as a thing while at the same time maintaining the desirable features of capitalism, e.g., reward mechanism and incentives. UBI builds on that to, to be sure, but it certainly reflects an idea of common ownership of some commons on which rents are collected (and, if its going to work at all in an increasingly capital-intensive and less-labor-dependent economy, those "commons" are going to include a share of superficially privately-held capital -- i.e., the means of production -- so its pretty hard not to see the direct connection with socialism.)
I think the fact that the use and disposition of the "superficially privately-held capital" is determined by individual private entities is actually fairly significant, even if they're losing a cut of the revenues. It strikes me as similar to the way in which paying taxes on our labor does not make us "slaves". Whether the lack, in this sense, of "state control of the means of production" is enough to make it "not Socialism" seems to be something people could reasonably disagree about.
