Interesting! Whenever I have to migrate code, I normally change only the files that are being actively developed. Trying to migrate tons of code at once can mean you do it without fully appreciating the nuances of what the code is doing (since you're not really working on that feature, just migrating it), and digging up old code just to convert it to a new language is a recipe for breaking things that were working fine before.
It sounds like they did consider that kind of "change it when you touch it" approach, but decided that it would be "exhausting and inefficient". Still, I'm surprised they went with a mostly automated solution. A good conversion doesn't just need to change the syntax but the mindset and idioms of the code. I'd rather have idiomatic Java than machine-generated Kotlin.