First of all, that isn't true. It depends on your jurisdiction but handshake agreements are enforceable in many circumstances[0], although proving details can be difficult. "If it's not written down, it's not a contract" is conservative advice, and generally good advice for how you should treat agreements offered by others. But it is not the legal definition of a contract, which can include verbal or even implicit agreements. That's why so many contracts have the text "the terms of this contract supersedes all prior agreements, written or otherwise."

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.