I think there are two reasons for the findings of that study
1. many plug in hybrids are company leased cars and their drivers may not be motivated to charge them and drive in electric mode.
2. apparently many types of plug-in hybrids are not capable to maintain highway speed on battery so the petrol engine needs to run much of the time.
So a plug in hybrid may be about as good for the environment as a full EV, but it depends on the type and on how it is used.