A factor that convinced Facebook’s security team that this was appropriate, sources said, was that there was an upcoming release of Tails where the vulnerable code had been removed. Effectively, this put an expiration date on the exploit, according to two sources with knowledge of the tool.
As far as the Facebook team knew, Tails developers were not aware of the flaw, despite removing the affected code. One of the former Facebook employees who worked on this project said the plan was to eventually report the zero-day flaw to Tails, but they realized there was no need to because the code was naturally patched out.
From that article.
Still very sketchy and would need to look further into the legitimacy.