Not necessarily? The correlation is observed, and there may be causation, in one, or both, directions, that is all we can conclude from the study. The causality doesn't have to be linear with the amount ingested, few things are linear in biology. For example, one could imagine that a certain amount blocks the development of a particular region of the brain. Ingesting more does not necessarily blocks it "more".
There is no evidence that this happens, but it is worth exploring, because schizophrenia is a hell of a condition, and is still largely misunderstood.
I'm saying we cannot interpret this research study the way people want to. Correlation != causation, and the results on this seem murky in general, as outlined how amount consumed does not affect likelihood of symptoms.
Comparing modern Covid research to this is... dishonest. We have a massive amount of data to parse and analyze with vaccine effectiveness etc.
Meanwhile, this is a single meta-study that doesn't make any strong claims.
2. This article in no way states that cannabis usage causes schizophrenia. It says there is a correlation, and furthermore the fact that both low and high usage having the same effects. That actually harms the case that it DOES, since you would expect the odds of it causing issues would increase with amount ingested.