Earlier quoted context omitted.
I call the fallacy a fallacy - it seems way to simplistic to me. There is obviously a fluctuation in work demand, for example all the seasonal work, but this fluctuation is not arbitrarily large. Take it to the extreme, 20 instead of 40 working hours per week - how would this not require twice as many employees, maybe even a few more because of reduced efficiency, to yield the same output? In essence this is nothing…
It's hard to put this very politely, but you're making a fool out of yourself now. Lump of Labour fallacy isn't crazy voodoo economics, it's incredibly well established. Read the Wikipedia article, then look through some of the references. Your concerns are amply addressed.
I suspect the only reason most people accept it is that they learned it at school so it must be true.