Different studies can show different results for a lot of reasons... Sometimes, it's just random chance. Other times, it's due to bad study design, or poor experimental controls, or deliberate P-hacking. There's no way to tell from the results of a single study.

But that's not what this study is for, anyway... This is a preliminary observational study to look for possible drug candidates that might impact COVID severity. It's only suggesting potential future directions of investigation, not advocating for immediately dosing everyone with SSRIs... Medical policy changes won't happen until a much larger body of evidence has been established.

It's the nature of science that not all experiments produce correct results. Sometimes, we get it wrong--but that's why we have to keep repeating experiments, and trying different variations, before we trust a conclusion.

In the meantime, though, medical decision makers will weigh the costs vs benefits of proceeding with an experimental treatment... If the risks are low, and the need is great, they'll probably let the door open to more experiments. Most people seem to think that's a pretty sensible way to do things.