UBI has been defended by capitalists as a way to stimulate the economy

Only in the marxist 'people who own companies' meaning of the word, not the ideological 'people who support free markets' meaning of it. Yes, many business leaders and wealthy elites hold (or purport to) 'champagne socialist' luxury beliefs.

Incidentally, ideological capitalists do not support government stimulus of the economy either. Corporate handouts are just as noxious to us as welfare.