Not what the author of React says:

> Yes, the first prototype of React was written in SML; we then moved onto OCaml.

> Jordan transcribed the prototype into JS for adoption; the SML version of React, however great it might be, would have died in obscurity. The Reason project's biggest goal is to show that OCaml is actually a viable, incremental and familiar-looking choice. We've been promoting this a lot but I guess one blog post and testimonial helps way more.

https://news.ycombinator.com/item?id=15209814

Sure, a prototype of an idea that would eventually become React was rewritten into JS to create the initial seed of the software that would eventually be called React.