Many folks in the security community might suggest a) An oblique warning publicly like "There exists a security problem with this; I have mailed the devs" b) actually mailing the devs c) waiting for confirmation of fix or a reasonable time and only then d) tar-and-feather. The term-of-art for this is "responsible disclosure." This incentivizes people to fix things quickly and preserves the reputational value of break…
A policy of so-called responsible disclosure is a reasonable approach to take when dealing with an established product/service that contains a minor vulnerability, something potentially dangerous but unlikely to be exploited in the immediate future with serious negative effects.
In this case, we appear to have a new project run by people who don't know what they're doing, with a glaring vulnerability that had presumably already compromised 80+ people's sensitive credentials and in turn who knows what other sensitive information. Bringing it down as fast as humanly possible and loudly so no-one else gets damaged in the meantime is entirely justified in a case like this.