This is true, and we provide that advice in the blog post we link to under the Embargo Strategy section:

"Can journalists break embargoes? Yes, some journalists notoriously will not honor embargoes. There’s an easy solution though — only include information in the pitch email that you would be ok with a journalist publishing. Once they agree to the embargo, you can share the rest of the story."

However, we've worked on hundreds of campaigns under embargo, and a journalist has never broken one. With early-stage startups, you can't really afford to be cagey with your news, as response rate will be so low anyway. You have to provide the lede upfront to get journalists interested in the first place.