This is talking about the identity of fish though, I'd guess that this is not talking about the toxicity of the meat. The article does mention that they are certain species which are known for higher toxic chemical contents, so that can be an issue.

However, isn't the toxicity of the meat is a separate issue? I have no idea how the FDA tests for that (sample testing?) but it probably isn't just discrimination on the species of the fish.

It's highly correlated to species of fish; some species have much, much higher levels of toxins such as mercury, depending on where they live, where they are on the food chain, etc. You can't separate the two.

It matters because its the consumers responsibility to know what they are buying. Obviously for a consumer to do their due diligence a product must be what is claims to be.