That is false, though.
In the manga, they are cyborgs, which means that they are humans improved by mechanical parts. They were later retconned into being modified by artificial cells. This change helps to explain why their cells are special enough to become Perfect Cell, so it was probably for the better. In either case, #17 and #18 were never supposed to be machines like #16 or #17. They were simply enhanced humans. Therefore, they ARE capable of getting stronger.
However, the thing that makes no sense is how strong #17 become with so little training, especially when there was no indication of his potential being just that high. His case isn't as extreme as Freeza, though.