The author states: >'Similarly, a Java programmer must understand the runtime behaviour of the JVM, but until OpenJDK, the implementation language was irreleva

The author states: >'Similarly, a Java programmer must understand the runtime behaviour of the JVM, but until OpenJDK, the implementation language was irrelevant." Can someone say why did the implementation language become relevant with the arrival of OpenJDK? My java app should run on any JVM no? After all that was the raison d'être of Java no? Or is the author referring to something else?