Great-grandparent comment implied the only cost you need to consider is for unemployed people i.e. that it's free for those who are employed . That's wrong.
That's not at all what the great-grandparent comment said. It was talking about the aggregate cost of the entire system. And the great-great-grandparent was wondering "just where is the money going to come from?" but fundamentally it's not a problem. As long as the total basic income of a country is less than the total income of a country, the latter can be redistributed into the former.
By this argument, if the basic income == the total income of a country, there is still no aggregate cost. Maybe, but I'd argue that such a system would come with huge costs due to the massive increase in taxation.