"The used a wrong offset, as the parent poster indicated. They assumed a simple offset could adjust for the differences."
The parent poster didn't have any evidence for that statement; he just assumed it, because of the way that the data was graphed. Point out the place in the article where they say that they used a single offset.
From the section I quoted, they seem to be aware of the problem. They could have easily adjusted each data set individually for time zone, then taken the average of the sets to make the graphs shown. They do say that they're graphing average European and American traffic.
2) They assumed a simple offset could adjust for the differences.
A simple offset isn't enough:
The (by far) majority of Europeans are in a single timezone.
The majority of internet-using Americans europeans are in two timezones with a 3 hour time difference, and very few users in the timezones inbetween.
This makes a significant difference. To compare to Europe, they should split US into 2 "nations": east-coast and west-coast.
Their method was wrong. Not how they executed it.