He briefly mentions hot reloading and declarative UI at the start, but it's drowned out by his ranting on JS and doesn't seem to earn it any points in the end. But those two are huge boons for UI development, to the point where it's worth putting up with JS's flaws as a language. Flexbox and media queries make CSS pretty decent for layouts. Dynamic languages are a double-edged sword - for UI you often want something up and running and useable to see if it actually feels right, without worrying about interfaces and types too much. Objective-C and Java can feel clunky in comparison.
Are you really trying to say that React Native makes Java not relevant for Android and ObjectiveC/Swift not relevant on iOS. Maybe you are correct if you want to build Hello World application, when you will be needing something more optimized sooner or later you'll have to rewrite it in Java/Swift.