I'm not an expert in the SN protocol, but I assume they are right about it. There are some disadvantages to low purity SN, as I see it:
1. It's harder to prepare the dose if you don't use pure SN. You would need to make sure you know what the exact purity is and you would need to calculate how much you need to use. If it's (almost) pure, it's more straightforward, you can simply follow the protocol.
2. What's the other component? If it's toxic, it could make you suffer a lot. Or maybe it could make you vomit. Or it would affect your body in a way that it would "save" you, or would make our death slower. Or maybe it is dangerous, or it's corrosive if you drink it, or it simply tastes way worse. It's not desirable to drink unknown chemical products, even if you are supposed to be dead soon.
3. Even if the other component is harmless, you still need to drink it. If you use low purity SN, you would need to drink more water or drink a more concetrated dose. That means you are more likely to vomit, or to be unable to drink it, or it could affect the way your stomach absorbs the SN.