In JDK-8027959 G1 introduced early reclaim for humongous objects that have no references.
The check whether a humongous object has no references is performed whether the object is not an object array.
This automatically exclude those.
However it is possible that regular non-array objects are humongous: while there is a limit on the number of members of a particular class, subclasses may be larger. That means that regular objects can be humongous.
Which is a problem because regular objects may contain references that might be missed if they are reclaimed during a concurrent marking cycle.