The tricky business is defining effectiveness (and the target population). This drug was not effective at the specified clinical end-point for one set of patients. But it did show promise for other end-points (production of a normal dystrophin transcript).

For complex diseases with a range of severities, failing the specified clinical endpoint is not the same as selling dyed water.

Is production of a normal dystrophin transcript a real clinical endpoint, or just a biomarker that's supposed to correlate for one? It's not immediately obvious how normalizing production of a protein involved in muscle function is beneficial, if it doesn't actually improve muscle function.

Absolutely. But DMD has a spectrum of severities, and it seems possible that for some patients, producing some normal transcript would make a real difference, so that until more people are treated, we cannot know when more normal transcripts make a difference.