* Facebook Messenger came out on Android in 2011 [0]
  * React Native came out in 2015 is written in C++, Java, JavaScript, Objective-C, Kotlin [1]

[0] https://en.wikipedia.org/wiki/Messenger_(software)

[1] https://en.wikipedia.org/wiki/React_Native

So, even if they did use React Native, they still have 4+ years of code in the original language; and, React Native doesn't stop the use of Java

I'm pretty sure wg0 was being facetious.

Most importantly, why was Threads not written in React Native?

Because Instagram wasn't, and Threads is pretty much a copy pasted Instagram codebase (for good reasons).