The “volunteer” comment was rebutting the idea that there would be no productivity in a society where money is irrelevant. UBI is not a replacement of compensation for work.

Presently, the first ~$16,000 you earn through labor are not gainful. That income provides your basic subsistence. You pretty much have to spend that much just to keep yourself alive in our economy. The remainder of your income is consider “gainful”.

UBI doesn’t replace compensation for labor. UBI provides that subsistence income. UBI means that every dollar you earn through your own labor is gainful. Your subsistence needs for the week are met before you leave for work on Monday.