At Facebook, we use Flow for type safety. We didn't want to have to introduce support for the existential operator until all of the details were more finalized.
Until then, we needed to have something that required minimal work (e.g. changes to the parser) that could tie us over until the language had proper support.
Also, we've been using this before the existential operator was formally proposed. (I should've probably got off my lazy butt to publish this sooner, but oh well.)
The plan is indeed to codemod this to the existential operator when it becomes available.