He has a waaaaaayyy different definition of "secured" than I (and I think the SEC) do. That tweet is going to cost Tesla and Musk big time.
However, a handshake agreement could be valid between individuals depending on the context.
In the US, the general law is that verbal contracts are worth the paper they are written on, which is to say, not at all unless other facts/circumstances support the existence of the verbal agreement and its specific terms. Otherwise anyone could claim to have a contract with anyone else.
For example, I claim to have had a verbal agreement with Mark Z for the development of a social networking site owned by me but built by him. In the absence of any other facts, no court would agree that such a contract exists. However, maybe I show copies of emails where we discussed the idea for a social networking site before it launched. That's some evidence that a verbal agreement might have existed, but not as to what the terms were. Most likely, the parties would settle without going to trial, simply because Mark wants to proceed with the IPO without the sword of litigation hanging over his head--even though he would very likely win. The marginal cost of winning at trial is not worth the much larger cost of the harm to the IPO. (This is very loosely based on the Winklevoss saga.)
Specific types of transaction and certain value thresholds require memorializing.
Facts and circumstances are required to support verbal contracts, but in general verbal contracts very much are enforceable.
There's a reason I brought up the falling apple example. Verbal contracts are enforceable--if they satisfy the same requirements as written contracts. On top of that, the terms of a verbal agreement must be corroborated by other evidence outside of the verbal agreement itself in order to survive litigation--and the trend in the US legal system is to require more supporting evidence.
However in this case all I'm saying is that a handshake agreement among reputable parties is enough to use the word "secured", whether enforceable or otherwise. It is common parlance to say that you've "secured an investment round" once a major investor has verbally agreed, presuming the final contract looks like what they discussed.
[0] For example, if a major investor said they would invest in a Series B at price $X so long as the terms matched at least the same terms as the Series A, other than price, I might then decline investments from others before there is ink on paper. If the investor then pulls out and I'm unable to get a funding round because I've already flip-flopped on other investors, the bankruptcy of my startup might attempt to take the first investor to court for violation of a handshake agreement. If the details could be proven, and especially if there was ill intent (e.g. they never intended to invest, just to torpedo the deal), they could be found in violation of a handshake agreement. The example of fraud is not required, legally speaking, but is typical of the cases you actually see.
It sounds like exactly what I'm saying...other (nonverbal) evidence supports the existence of the verbal agreement.
Saying that a verbal agreement is a contract is like describing gravity as an apple falling from a tree. It's technically true but ignores the complexities that go into what can make a verbal contract enforceable.
(And I say this based on actual practice litigating contracts...)
If you are a contracts lawyer, then you are doing a serious injustice to the profession by saying contracts do not include verbal agreements.
(And I say this based on actual practice litigating contracts...)
But a verbal contract you can prove is enforceable just like a written contract (with some important exceptions covered by a state's statute of frauds). Even in the case of conflicting stories, a jury can believe one side's testimony over the other.
Elon's problem here is that "I left thinking we could get a deal done if we did some other stuff that could get a deal done" is not a statement about an agreement. The Saudis don't have to honor his private prediction.