What do you mean by "optional"?. You are including it [2] [3] [4] as one of the options (not re-implementation of the original paper [1]) and it is one of the dependencies [4] (Prophet [5] is an open-source library built by facebook research).
I think we should distinguish between science/open-source and policies when we mention open-source projects.
If I put out a buffet where some items are vegetarian and some are not, I don't advertise it as a vegetarian buffet with the excuse that the nonvegetarian foods are optional.
That’s actually exactly how vegetarian buffets work.
If you want to avoid Facebook with this library you can. That may not be the case with every library so folks who care about such things appreciate the callout. If you don’t care you don’t have to.
As a side note when in Brazil I felt like "vegetarian" food meant "just little bit of meat" and "caipirinha without sugar" meant "just don't mix (the layer of sugar at the bottom)" :) Did love it though.
Edit: it's important to note that this is an alternative to Facebook Kats, I should've probably added this in the title and that's my bad