The sale contract with the dealership probably has some language like "contingent on final approval from the loan company," and the lender probably has their own rules about only approving loans for people with a valid driver's license. The dealership should know those rules, but I think this person is getting caught in the very unfortunate but definitely real bureaucratic nightmare where it's often your own responsibility to know that someone who you believe knows the rules better than you is wrong.
That said, everything up to the last few sentences sounds like they're trying to do things the right way, but if they try using One Weird Trick(tm) to keep possession of the car they're probably going to cause much bigger problems for their future self.
First they sent the lender some form that they didn't understand, claiming that the form itself was sufficient payment for the loaned they'd agreed to pay. The lender sent back a letter that probably said something like "no, you have to pay us real money like we agreed to" and the person responded with more forms that they don't understand instead of paying their bills.
This very predictably led to their car being repoed, since they weren't paying the loan, because they have been lied to by the people in this FB group who think notaries (people who have been licensed to confirm "this was signed by the person who's name is on the the page") and certified mail (the post office got a signature from someone to prove it was delivered, nothing else) have magic powers to make a document legally binding.
The entire idea is based on the self-contradictory argument that laws can only apply to YOU if you consent to them, but any and every oddly worded clause from any law or court decision, from anywhere and any time in the past, binds everyone else, as long as you know the right magic words to invoke.