But the security does exist I think; they're just not yet traded on a public/liquid market. I've received ISOs in two different companies that were privately held. Those were definitely options, definitely on securities that existed, and those became publicly valued and liquid only after a purchase by a public company (in one case) and an IPO (in the other).
It's entirely possible that I'm missing a subtlety, and I'll follow the discussion to learn what that is.
The subtlety is the Safe converts into a shadow class of securities with rights similar to the next round of preferred stock that the company issues. So those securities are not yet issued, their terms will be negotiated when the company raises that round.
In that case of your ISOs, those would be tied to the existing common stock of the company. While the actual shares that you might exercise on aren't yet issued, the class of stock itself exists and the terms are set.
It's entirely possible that I'm missing a subtlety, and I'll follow the discussion to learn what that is.