There seems to be a much more obvious explanation as to why people are assuming it used to be React Native. The article says:
> > Compared with the previous iOS version, this new Messenger is [mentions improvements].
> > We accomplished this by using the native OS wherever possible […]
That seems to imply that it wasn't native before and they got the gains by switching to native. And since Facebook are well-known as the creators and biggest users of React Native, it's not a big leap to make the assumption that that's where they were coming from.
[Also, in case there's any confusion please note the distinction between "native", meaning it is compiled to native code; and "React Native", which is a cross-platform framework that is not compiled to native code. "React Native" is a horrible name for something that is not native.]
If you want to go deeper or have more specialised requirements, then you might need a deeper understanding of the native platforms though.
Beyond "Hello World" all of them required me to get my hands dirty, at which point I was better off just doing C++ with native views, standard out of the box tooling available across all SDKs.
Nowadays I rather go for PWAs, or plain mobile Web, unless it is something very special that requires OS APIs or hardware access.
I guess many native developers feel negatively towards it because a lot of companies will be asking "why aren't we using React Native instead of building two separate apps?" which I can imagine feeling somewhat threatening towards your area of specialisation