In the context of discussing the library's marketing/presentation I think the the branding is also fair game. It's also a little odd how at the bottom of the page it gives credits to the image as a non-commercial license. Personally I would recommend avoiding any licensed assets in such a project all together (when possible).
But at the end of the day if you build and release an OSS project you should get to do whatever you want with it, like the use of emojis that was discussed in the postwoman.io project that came up last week.
My opinion is who cares, we should be discussing its technical merits first and foremost on a forum like this.
I think more projects should feature flowers. They're nice. I feel confident O'Reilly would back me up on this one.