Earlier quoted context omitted.
Communication is hard. If they said "Takes 50% to 200% more time" it would have been clearer
What? No, that's confusing enough it's almost hostile. The fact thst your math is wrong is proof enough. 50% to 200% more time is 1.5x to 3x slower. I don't like how it was worded by the author. But all you've done is essentially invert the wording while making the math MORE difficult in the process.
I think they are correct but it is easy enough to misinterpret that it is not a good way to phrase things.