When we say OpenJ9 we don't mean OpenJDK 9.
To clarify there are two Java JVMs(the bit that actually reads and executes the Java bytecode) in the market right now:
HotSpot, made by the OpenJDK community, and
OpenJ9, made by IBM
Both are implementations of the Java JVM Specification, and as long as you use Java correctly any code you write should work on both.
However, Forge does not stay within the lines. Forge leverages implementation-specific and internal utilities of HotSpot to do its black magic, meaning it won't work on OpenJ9 that is different internally.