Both the Community and the Ultimate editions of IntelliJ use JRE 8 to run themselves. However at least the Community edition needs both JDK8 and JDK6 for build and development of itself. At least there is some code that needs to be compatible with the JRE 6 API. This is according to the GitHub of the community project. Please read the "IntelliJ Build Configuration" section:
Why is this requirement still relevant? Or maybe it's already not relevant and that documentation needs to be updated? Anyway Java 6 is too old, why to keep any code of IntelliJ be limited to Java 6 or to its old API?