FollowReferences function with null initial_object traverse objects starting from "heap roots". Hep roots includes "set of system classes", and the classes are reported with reference kind JVMTI_HEAP_REFERENCE_SYSTEM_CLASS.
Actually a lot of non-class objects are reported as JVMTI_HEAP_REFERENCE_SYSTEM_CLASS - java.lang.Module and [Ljava.lang.Object;

To reproduce:
implement JVMTI agent which calls FollowReferences with jvmtiHeapReferenceCallback:
heap_reference_callback(jvmtiHeapReferenceKind reference_kind,
                        const jvmtiHeapReferenceInfo* reference_info,
                        jlong class_tag,
                        jlong referrer_class_tag,
                        jlong size,
                        jlong* tag_ptr,
                        jlong* referrer_tag_ptr,
                        jint length,
                        void* user_data) {
  if (reference_kind == JVMTI_HEAP_REFERENCE_SYSTEM_CLASS) {
    *tag_ptr = 1;
  }
  return JVMTI_VISIT_OBJECTS;
}

And then call GetObjectsWithTags for objects with tag 1.
returned array contains a lot of non-class objects