I would say it's pretty basic common sense, not to publicly announce ANYTHING that could immediately affect millions of people. Unless he's just a sociopath.
From his Twitter account, he's not just some layman stumbling across it.
If that were true, then the security community wouldn't have spent years fighting about whether responsible disclosure was the right approach. That's for people who actually understand this stuff. It's unreasonable to expect an outsider to derive it all on their own from first principles.
So someone stumbles upon a lost cache of chemical weapons. Rather than reporting to the authorities, they post its location on Twitter. That's called just using your brain.
You're coming at this from a position of knowledge and assuming everyone else knows as much as you do, or should be able to figure it out in short order. That's not how it works. It's really hard to see how other people might think in situations like this.
Security though obscurity is no security at all. Don't you think the people living around the chemical weapons should be informed too so they can take precautions to protect themselves?
