IMHO, it is very likely just a rumor. Musk is quite difficult guy to work with/for and the LAC management teams seems to be not that stupid to sell the Thacker Pass fast. 

At most, they may announce a supply contract, but since Thacker Pass will not be online before 2025, what would be the point to do so? Now I am talking business, not PR. Of course any TSLA related announcement would push LAC into new highs.