THC-O products are in a particularly precarious legal situation. Last week, the DEA stated that THC-O products are federally illegal because the compound is not found naturally in hemp. However, the DEA recently confirmed by letter that THCO is a controlled and illegal substance under federal law. It seems that the point here is that THC-0 has ALWAYS been illegal.
So the fact that you send a letter from the DEA to a lawyer expressing your opinion that it is illegal does not make its legality official.