It is illegal to abuse your access to certain things in order to squeeze competitors, if Tesla actually has contracts that would secure enough Lithium to have negative effects on competitors (no idea if that really is the case), abusing this position would expose Tesla to all kinds of legal troubke caused by this anti-competitive behaviour. Lithium supplier might then be legally obliged to supply Tesla's competitors at the same conditions as Tesla if not enough Lithium is availavle to fulfill orders. Working with Li suppliers to starve competitors of Li deliveries would even be worse. Given Musk's, and Tesla's, cavalier attitufe when it comes to rules and regulatuons, I wouldn't be surprised if they tried so.

Source: More compliance trainings I can count and enough real life experience with monopolistic raw material suppliers to know that abusing market position to hirt competitors is a really bad idea.