From line 1122:
// don't load packages that contain editor only data in builds that don't support that and vise versa
if (!FPlatformProperties::HasEditorOnlyData() && !(Summary.PackageFlags & PKG_FilterEditorOnly))
{
UE_LOG(LogLinker, Warning, TEXT("Unable to load package (%s). Package contains EditorOnly data which is not supported by the current build."), *Filename );
return LINKER_Failed;
}
if (!FPlatformProperties::HasEditorOnlyData() && !(Summary.PackageFlags & PKG_FilterEditorOnly)) expression decodes into this:
if (!false && !(true) ----> if (true && false) -------> if (false)
But message clearly says that “Package contains EditorOnly data which is not supported by the current build”