That was their initial problem.
Upon further reading, you will find that they found a solution in dealing with Lists with React Native. My initial comment had the answer - they brought in a component that visualizes lists from their web platform, and it worked seamlessly in React Native.
So no, while they initially implemented their core chat view natively, they are using full blown react native as of right now.
>The result was a new component we called <FastList>. The team intends to merge these together for cross-platform use and open source it for the community. With that we removed a lot of memory allocations and were down another 70-90ms of render time. We could now scroll the channel members list as fast as we wanted and it kept up admirably.
So no, while they initially implemented their core chat view natively, they are using full blown react native as of right now.