In a way this is even more worrying (knowing that the Mueller report release time was widely known beforehand).
It's probably "safer" for Facebook to make the announcement with the guaranteed disruption of the Mueller report release afterwards, than it is for them to release it after the report is out, since they can't predict how long people will be focused on the report. I can imagine a conversation along the lines of what I just wrote happening at Facebook, and that's really a distirubing level of coordination to hide a serious incident.
Facebook announced this before the Muller report was released.
The Facebook security post was updated April 18, 2019 at 7AM PT [1].
The Muller report was released "Thursday morning, shortly after 11 am [EDT]" [2]
[1] https://newsroom.fb.com/news/2019/03/keeping-passwords-secur...
[2] https://www.vox.com/2019/4/18/18411966/mueller-report-releas...
Disclosure: I work for a big tech company but not Facebook. All opinions are my own.