This isn't actually any kind of "theorem".
Besides the criticisms here and in the comments on the original site, the author is completely ignoring the fact that there are also people who would choose to work under UBI who don't work now (because that would mean the total loss of existing benefits). Would that be enough to counteract the number of people who chose not to work at all under UBI? I think it would. Whether that's true or not, ignoring it completely (as the author does) is engaging in a straw man argument, not a "theorem".
Under a properly-run UBI system, a person would never be worse off economically by choosing to work.
I understand the argument that there are people who would just watch TV all day. Yep, those people exist, but, IMO, most of those people are already doing that.
