Real world conditions vary from standardized laboratory tests. Emissions being higher on the road is not a surprise. No doubt GM tuned their diesels specifically to meet testing requirements.
If the EPA had something on GM, we'd have heard by now. There's no evidence that GM broke the law like VW did by disabling emissions controls while on the road.
If you read the some the article (many exist) lawsuit CLAIMS there is evidence...I'll cherry pick from the article, no need to thank me:
1. "It said GM used at least three "defeat devices" to ensure that the trucks met federal and state emission standards"
2. "According to the lawsuit, "on-road" emissions testing conducted for the plaintiffs found that Duramax-engined trucks produced nitrogen oxide pollutants two to five times higher than allowed"
3. "Germany's Robert Bosch GmbH [ROBG.UL] was also named as a defendant for having allegedly helped develop the defeat devices, in an "unusually close" collaboration with GM."
Remember the EPA never "caught" VW, a $50K grant was given to West Virginia University to road test diesel cars emissions, that's who caught them...
Odds are other automakers have and/or are currently using defeat devices, they just may be harder to detect even when on the road...