The cynic in me wants to believe that it's a way of deflecting blame somehow, to make it seem like they did their due diligence but were thwarted by something outside of their control. I don't think it holds. If you use an open source library with no warranty, you are responsible (legally and otherwise) to ensure that it is sufficient. For example, if you break HIPAA compliance due to an open source library, it is still you who is responsible for that.
But of course, they're not claiming it's anyone else's fault anywhere explicitly, so it's uncharitable to just assume that's what they meant. Still, it rubs me the wrong way. I can't fight the feeling that it's a wink wink nudge nudge to give them more slack than they'd otherwise get. It feels like it's inviting you to just criticize redis-py and give them a break.
The open postmortem and whatnot is appreciated and everything, but sometimes it's important to be mindful of what you emphasize in your postmortems. People read things even if you don't write them, sometimes.