The JetBrains claims are likely very accurate. Azul observed closely as the IDEA team at JetBrains worked through many OpenJDK fixes to ensure Retina support on Mac OS X works as it should. Any issues with subpixel rendering and sophisticated font display resulted in a significant set of work items for them, and I could believe they kept anything they could not upstream into OpenJDK back within their own runtime.
They do make one salient point in their posting here:
Inability to work with existing (original) fonts (Hasklig, Fira Code, Monoid) are caused by a deficiency in ICU text layout engine used internally by Java. This engine is officially deprecated and is scheduled to be replaced with Harfbuzz (not having a mentioned issue) in Java 9.
The HarfBuzz replacement for ICU is captured in JEP 258, which is here http://openjdk.java.net/jeps/258. Follow the trail through JIRA and you’ll see this was fixed as integrated into OpenJDK 9 build 96. The Zulu 9 Prerelease 7 is up to build 131, so it should have Harfbuzz ready to go. You can check whether PhpStorm Editor on Zulu 9 Pre-release 7 for Windows gives you ligatures, and if not go back to JetBrains to see if they have or plan to have an EA build that supports Java 9.
Hopefully this helps.