So, this might be a case of differing sources of information. I've never actually heard much emphasis out on the life outcomes angle, it's always been in the delayed gratification techniques part.
It's a case where I'd hesitate to use the phrase "discredit" because they actually did demonstrate what they were looking to show with the marshmallow test, and their findings do correlate with life outcomes: specifically delaying gratification and managing frustration. It's just that childhood marshmallow skills don't imply anything about study diligence in teenagers.

It would not at all surprise me if people used it to create a misleading narrative, I just haven't actually seen it personally.