They aren't out of their minds when they say FC of 0.05 can kill algae, they are simply talking about something that is irrelivant in practice. FC of 0.05 with CYA at 0 has the same active strength as FC of 1.5 with CYA of 30. Which is to say that it can kill algae if there was any way of maintaining that level. The problem with really low levels is that they get used up almost instantly and then there is nothing left and no way to replenish the FC quickly enough. The first cell of algae would die, but if a second cell of algae fell in the pool there wouldn't be any FC left and it would do just fine.
Algae will die from chlorine with concentrations as low as 0.05ppm.
It's the whole truth but the catch here is, you have to be able to maintain that 0.05ppm Chlorine without it going to 0. I suppose they do not mention how long it will take or how strong are the algae outbreak. Very misleading but I belief in it though very impractical (correct me if I'm wrong).