My guess is that the Facebook employee thought it was simply not worth using an untrusted (22 stars) library like this for the project. Maybe there was some additional feature where they just didn't want to deal with asking the original author of the idea and wanted to build themselves, but I'm guessing because the project is going to have significant usage of this (and probably growing as more complicated use cases come up), in house control is easier to manage (eg. imagine the case where the repo goes offline or something). Also, it doesn't look like this was 'Facebook inspired', but done on the developer's own time. Likely some weekend project because they were bored. I really don't think this is done in malice like some other comments hint at.