DXM isn't codeine. It will still produce a narcotic high if you take enough of it, but requires a lot more of it. The problem with using OTC cold meds is that they usually have acetomenophen (Tylenol) along with it. By the time you get enough DXM to get high, you've also gotten enough acetomenophen to cause liver damage.
Interesting. I recall that the autopsy found liver damage:
This link has a screenshot of the autopsy report, describing the liver.