Should have been handled better, but some context is necessary:

If your name is associated with a startup in a visible leadership position you will get mass-spammed from people claiming to have discovered critical vulnerabilities in your system. When you engage with them, the conversation will turn into requests to hire them for their services.

So the CEO handled it poorly, but it's also not a great choice to withhold the details of the vulnerability in initial contact. If the goal was to get something fixed it should have been included in an easy-to-forward e-mail that could have been sent to someone who could act upon it.

Anyone who works with security or bug bounties can tell you that the volume of bad reports was a problem before LLMs. Now that everyone thinks they're going to use LLMs to get gigs as pentesters the volume of reports is completely out of control.