It's essentially mandatory unless you provide better terms on your own. If the unions want a CBA in that case, it doesn't change a single thing. You sign and nothing changes. Tesla claims to provide better than the CBA but openly refuses to sign which doesn't pass the smell test.

Tesla wants to punish anyone who joins a union which is super illegal. We're talking prison time if they don't back down. They can't take away the stock benefit when IF Metall wins. They've put it out there, and if they take it away because of unionisation, they'll lose. It's a huge fucking no.