The article actually addresses Kotlin. They'd love to switch to it but they just can't do it overnight because they have so much mission critical Java code. So, this is a stop gap solution for legacy code. They published another article some time ago how they are switching to Kotlin: https://engineering.fb.com/2022/10/24/android/android-java-k...

Migrating millions of lines of code is a non trivial effort. They'll be stuck with bits of Java for quite some time. So, this helps make that less painful.