The article provides reasons and findings why productivity can be doubled by switching from Java to Kotlin. Here are some example languages that were released after 1970 that could more than double productivity compared to some predecessor:
1972 - C
1978 - SQL
1980 - C++
1990 - Haskell
1991 - Python
1993 - R
1995 - Ruby
1995 - Java (managed languages simplified many things)
2001 - C#
2003 - Scala
2009 - Go
2010 / 2011 - Kotlin (version 1.0 released in 2016)
I'm sure I've missed others but the point is to analyze the reasoning and logic of benefits prior to making overarching statements that dismiss improvements. Experts in each of these languages could definitely make a case for their respective language compared to some predecessor since all of these languages introduced many benefits. The article looks at the benefits of Kotlin in comparison to Java.