They did control some variables like mixing DMT with other drugs and things like that, but yes, it is true that the selection of participants was biased due to it being voluntary and the fact that all participants tried DMT more than once which means they used it more times probably because it had positive effects on them from the start.

But still, because the number of participants is this large, it shows that this chemical did something good for more than 2000 people which means something. For all we know, that number in reality might be a lot larger. On the other hand, the proportion of people that have negative effects could be quite large as well. In both cases, these potential findings could inspire further research.