That article doesn't say anything about selling a mine, it says the only mine was already in China. China bought an American magnet manufacturer which had been using Chinese neodymium to make magnets. It doesn't say anything at all about extraction.
Also, nobody forced US companies to sell those assets.
Originally there was the Mountain Pass mine, and China setup their own mines while also building out the refining and processing capacity. Until recently MP Materials was shipping all it's output to Chinese refiners.
The history and asset ownership is also quite convoluted. Try this Wikipedia article:
The article explicitly says that the magnet manufacturer was bought by a shell company owned by chinese interests, which lied when they pledged to develop the activity.
Not that it's very surprising (US companies routinely do this as well, especially in Europe), but in this case it had clearly an ulterior motive.
Also, nobody forced US companies to sell those assets.