I have read that argument and seen it refuted; seeing as there are no alkloids common in opium in amounts anywhere near that of what is found in tar; indeed only being varying degrees of acetylated morphine (heroin, 6-MAM poss. 3-MAM), so can compete in potency with equal purity heroin of all the usual steps in its creation process due to higher 6-MAM content than otherwise had in di-acetyl-M.
e.g. 75%-80% 6-MAM as total content of DEA detained examples of tar heroin wouldn't mesh with it being opium that has simply been acetylated.
We would instead see thebaine, papaverine, reticuline, noscapine, codeine (or acetylated versions thereof) in much greater amounts. Possibly not an injectable preparation either, as I've not heard of opium tincture, deodorized or not, as being safe to inject.