This paper seems to me to be saying that the experimental results suggest that allelopathic chemicals may be present in some aquatic plants. I don't see how the results can be interpreted to go beyond that. I think it is good scientific technique to do things that concentrate the effect of chemicals, apply conditions most favorable to seeing the results you are looking for. But, then you have to accept that all you have done is show the possibility that your result applies in the real world, but you haven't come close to proving it. As I understand it, a lot of preliminary cancer research is done in that way.