Can anyone start Tesla-compatible car production?
Anyone attempting to go into business faces something called "barriers to entry." Barriers to entry can be laws that must be complied with, financial or technical. The fact that Tesla has opened up their patent file to anyone who wants to use them only eases those barriers slightly. You still have to build a plant, equip the plant, secure suppliers, do your marketing (remembering that you are always going to be chasing Tesla, and of course all of this takes money.
I don't know that "anyone" can start Tesla-compatible car production, but someone can. The question is "is it worth the financial risk?"