It looks like that study checked the effects of short-form content addiction, rather than short-form content in general. Addiction can be caused by underlying factors, such as stress or depression, which are shown to reduce attention span so I don't think it really shows a direct causal connection. In fact, I think it's more accurate to say short attention spans cause short-form content rather than the other way around.

That said, excessive social media consumption can make stress and depression worse, I just think we're focussing on the wrong aspect of social media's effect on our mental health.