You can use latest Idea to write code on Java 1.4, I guess. The topic is about JRE used to launch Idea itself. You can use any JRE to launch your projects from Idea.
I realize that Hacker News skews toward a different industry segment, and it's hard for people outside of the enterprise to fully grasp its horrors. But believe it or not, expecting the current version of Java to be installed on a Java developer's machine is actually a pretty big shift!
I should note that this JDK 8 migration is for EAP releases (ie early alpha/beta versions of the next major release) and that all installers (windows, osx and linux) offered by Jetbrains come with its own JDK bundled in the installer. So if you happen to work in one of those corporate environments, if you manage to get permission to run this IDEA version, you also get the required JRE to execute the ide included.
Or is your question about how JetBrains expects to get permission from the pointyhair bosses to purchase the new IDEA version for their developers in the first place? If that's your question then it's not really JetBrain's business if megacorps decide developer productivity is not worth the money to spend on buying a new copy of IDEA.