You don't get to say it isn't. I wet on a 6 week NHS course for chronic pain, and my mental health due to being in chronic pain. Exercise was one of the foundations of that course, not to make us feel better, or to eliminate the pain, but to make our bodies healthier all round. The stronger our bodies the better or bodies are equipt to deal with chronic pain. Meditation, neurology, and psychology were the other foundations. Exercise was just as important as those 3.

Is your frustration that people say it is more effective or that they highly recommend exercise?

I'd be dead without my meds. People have no idea

It's preventative in the sense that it lowers the chances of getting depression for healthy individuals. And it's not just the act of exercise itself but its social effects from meeting and interacting with other people. And the overall boost in general wellbeing.

There have been hundreds of studies regarding exercise as a core treatment for depression, including the effectiveness of what kinds of exercises (asrobic, strength training, etc) are the most effective. Downplaying how well established this is (especially without voting a source) is irresponsible. https://pubmed.ncbi.nlm.nih.gov/38355154/

Not a cure but 1000% can contribute to preventing it.

Indeed, your results must be valid in order to be used as evidence. The requirements for valid results include repeatability and reproducibility. This means that you can repeat the study, and get others to do it, and you'll get the same or similar results over and over again.