Strong arguments can be made for the necessity of basic income, but as a concept it is weakly socialist. The ideal that people have some fundamental right to sustenance beyond what the market value of their labor provides as income is by definition socialist, given that the corresponding capitalist ideal is that, if you're not worth paying enough to live, you die.
The modern mixed economy that every Western nation has is called that because it is a mixture of capitalism and socialism.
Its true that BI is consistent with that mix, but that's not a counterargument to it being from the socialist side of the capitalist/socialist divide. (Of course, the utility of that divide and the breakdown of arguments over economic arrangements into "which side of the divide does that fall into?" followed by "if capitalist, good; if socialist, bad", or vice-versa, is dubious.)
If anything this is the pure form of the mixed capitalism/democratic economy/government model that every Western nation already has.
BI is a way of reconciling capitalism with the "post-work" work world we are moving into.