Only applies to single-junction cells. 68%-efficient solar panels are quite theoretically possible, they'll just be incredibly difficult to manufacture cheaply. The current world record for solar panel efficiency is 47.6%.
I agree, however the author is convinced about current popular solar panel technology.
Indeed even theoretically we can do 86% multi and 99% with quantum dots——in the current state of technology these remain science fiction considering the commercial manufacturing processes available.
Sure there are many novel solar technologies that subvert the SQ assumptions thereby avoiding the conclusion. However, the author of the parent post believes that existing technology today is sufficient.