I don’t see anything about a 15% mortality rate. Can you point to that specifically?

~15% need serious medical intervention. That's at the very beginning of the article I linked, the "severe" cases in the table.

Now under normal circumstances this intervention is not life threatening--that's the smaller "critical" category. But without access to common hospital equipment, the symptoms do become critical. Basically the patient becomes unable to breath normally, and without access to a breathing machine they'll suffocate themselves.

Most "severe" cases are treatable with access to a hospital. But if we peak at once and that equipment is all occupied by other patients... that 15% of severe cases become fatal.