Paragraph 8 defines chemical weapon as "the following, together or separately". Before looking at the content, you have to ask what that could mean. In saying "or separately", that means that if any one of the following is true, it constitutes a chemical weapon. Subparagraph (a) identifies toxic chemicals (it qualifies) but with an exception ("intended for purposes not prohibited under this Convention"). The problem here is that this does not identify the scope of intent – intent in development, or intent in use? At the level of historical development, the intent was allowed, but the specific application was not clearly allowed. So Nazi gas may or may not count, depending on whether "developmental intent" vs. "applicational intent" holds sway.
A bit of common sense tells you that "chemical weapon" cannot be defined in terms of developmental intent. Mustard gas, a notorious chemical weapon, was developed for uncertain purposes but apparently along the lines of "let's see what happens when we do this" (perhaps in 1822). Chlorine gas, another chemical weapon, was not even "developed", and was used as a general chemical agent for centuries. Phosgene, yet another chemical weapon, was synthesized in 1812 (for the purpose of scientific research) and was used in dye manufacturing – again, the original intent was not as a weapon. Sarin was originally developed as a pesticide... you can look at the long list of specific chemicals identified in the schedules here. We have to conclude that "original intent" is not the kind of intent referred to in the treaty, since to hold otherwise would lead to the absurdity that the convention applies to nothing. So the question is whether the specific use is allowed. (See below).
Subparagraph (b) adds "Munitions and devices, specifically designed to cause death or other harm through the toxic properties of those toxic chemicals": that is, it includes not just the toxic substances, but also the delivery systems with the proviso that the delivery system is designed to cause harm. It is conceivable that the Nazi gas chambers were originally designed to be permitted fumigation devices, though this would not be the case with the gas trucks. It is more probable that the specific chambers were developed for mass human extermination. Under this subparagraph, Nazi gas chambers were also "chemical weapons" under that definition. (The historical question is whether the chambers were literally unmodified, and I am voting that there were modifications, which is a historical question best taken up on History SE).
Paragraph 9 articulates a possible escape hatch, a propos "intended for purposes not prohibited under this Convention". In particular "Law enforcement including domestic riot control purposes", is a convenient excuse. "Riot Control Agent" is defined as "Any chemical not listed in a Schedule, which can produce rapidly in humans sensory irritation or disabling physical effects which disappear within a short time following termination of exposure", but "Riot Control Purpose" is not defined. Under a narrow literalist construction of 9(d), if there is a law prohibiting being Jewish, one might attempt to argue that by creating such a law, the regime has immunized itself against the accusation of using chemical weapons. But such a construction of the convention is ludicrous, insofar as any offending regime could simply pass a law allowing chemical exterminations, and then they would be immunized against accusations of violating the treaty. This too is an absurd result.
The particular problem with subparagraph (d) is that it does not specifically limit "law enforcement use" to defined "Riot Control Agents". Indeed, the treaty does not give a general exception to "Riot Control Agents", and that term is only used in connections with other prohibitions (Art. 1 para 5: "Each State Party undertakes not to use riot control agents as a method of warfare"; Art. X para 8(b), state parties may request assistance if "Riot control agents have been used against it as a method of warfare"). In other words, one could reasonably infer that the convention is not intended to prohibit use of Riot Control Agents for the purpose of (internal) law enforcement – mace is legal – but they cannot be used in warfare. That is not what the "law enforcement" exception says, though.
Just to cover all escape routes, we should turn back to the exception clause in (8): "except where intended for purposes not prohibited under this Convention". The convention does not actually clearly articulate any prohibitions nor does it specifically say "this is permitted", instead it defines "Purposes Not Prohibited Under this Convention" and then gives that list of 4 "not prohibited" uses. Since exterminating people is not one of the 4 "not prohibited" uses, we have to conclude that extermination of people is not allowed under the "except where intended for purposes not prohibited" clause.
All told, I conclude that both of the topically-relevant uses of chemical agents do constitute "use of chemical weapons", assuming the OPCW definition.
