The Alibaba version with 11AH specifies Samsung 22P cells. This is an old and cheap cell, but one that has stood the test of time (from about 2010). If you take 50 cells that cost around $2, put them in a case that has to be reverse engineered, and assemble it with a robotic spot welder, what should it cost?
It's an interesting question. Canon and Panasonic resisted knock-off batteries for their cameras, and to some extent they succeeded. But it's not very customer friendly. There's not a whole lot to an ebike battery, though people can argue the safety elements. I don't know when CNC equipment becomes cheap enough for companies to want to make knock-offs, but it is pretty soon.