I don't think you are getting it. If I have 'SomeBigClass.java' and do the conversion, and don't commit until I'm done making it 'SomeBigClass.kt', when I look at the history of 'SomeBigClass.kt', it will show as 'added' today.
If we do what IJ now does (commit right after rename, and then commit for conversion work), when I look at the history for SomeBigClass.kt, I will see the history of SomeBigClass.java as part of the history list.
If you don't care about that, then your approach is 'correct'.
If you DO care about that history, then IJ's approach is 'correct'.
Many users felt the latter was 'better'. You (and others will, too) disagree.
IMO, IJ should make it a flag, or prompt on conversion or somehow make it optional/visible to users. But neither approach is correct, and neither is wrong. Fundamentally all of this is working around an issue in Git's rename detection strategy.