I don't think it is. If it is, its a very new rule. Very, very new.
I believe that calling something "lutz" in marble-collector terms is usually meant to indicate the appearance, or look of a substance rather than the method or materials of its manufacture. If I say a transparent banded lutz, or an onionskin with lutz, most of you know what I mean. It has a quite different appearance from what is usually called aventurine of the same period (even though they both may be aventurine, produced in a very similar fashion, accidentally or not). When I see exactly the same sparkly-gold-lutz-effect on an Alley (and there ARE some, just not the one pictured) or -- gasp -- a Jabo, it's lutz, appearance-wise. If you want to drop the use of the word lutz altogether, fine. There may be valid reasons to do so, although it would eliminate a useful term from our marble vocabulary. But you'd need to drop it across the board. No Alley lutz, no onionskin lutz. Just banded transparents with gold aventurine.