There are loads of terrible companies around, so I'd be surprised if none of them had ever tried to intercept Blind sign-up messages before now, and if you're a tyrant CEO at one of said terrible companies, getting the emails forwarded to you is merely the next logical step.
The next paragraph in the text is a lot more interesting:
Some of the most commonly discussed topics on Blind are protected speech in the U.S.—pay, job terminations, critiques of workplace conditions—which we believe workers should be free to access and discuss.
What are the consequences (in the US) of a company blocking that?