It's not the case that more CO2 automatically means more plants. Plant growth has lots of components: water, light, nitrogen, etc. Adding more gas to a car's gas tank doesn't make it go faster, unless it's empty. Adding CO2 to the atmosphere doesn't make plants grow faster if that's not the limiting factor.

Instead, adding CO2 can cause it to metabolize differently. It will put growth into different components. In this case, it's a toxin that causes a lot of problems. That's news because people are going to have to deal with additional toxicity from this plant.

So this is novel information that people will need to respond to. That's not really conveyed by the journal article's title, "Recent CO2 levels promote increased production of the toxin parthenin in an invasive Parthenium hysterophorus biotype."

If that reads as feeding conspiracy theorists... honestly, what doesn't feed conspiracy theorists? In the lack of information they're free to simply make stuff up, or repeat things that have been debunked many times. There's just not a lot you can do about it, so I don't think it's worth taking them into account one way or the other.