They're converting Twitter fame into capital funding. Not having a product means you technically don't have an obligation to deliver anything in return for that capital.
I was under the impression that potential investors usually don't sign NDAs.
Correct. Or rather, I've never seen it. Some are even offended when asked. N=~150
Investors do not often sign NDAs covering "ideas", they will sign (and will often ask you to sign) NDAs with scope limited to investment terms and due diligence information.
Probably depends on how much leverage those seeking funding have.