Oes data and analysis present a good argument? Debating the research method of an article

This is the instruction:
Grandcolas, Rettie, and Marusenko (2003)article (is attacehed) examines the difference between surveys administered by paper and pencil procedures and by Web procedures. This paper is published in a refereed journal, and its quality is assured. However, any paper can be scrutinised and is open to criticism in part and improvements.

In the abstract the authors write: We argue that the differences found are attributable to sample bias rather than mode, and that the ramifications of low response rates in web surveys are more far reaching than has been recognized.”

-To what extent do their data and analyses present a good argument for this? Why? What might you have suggested to them to improve this argument? What other criticisms can you offer?
Consider in your synthesis the various perspectives on diversity and their meaning for leaders in organisations. (in the Easterby book chapter 10 that I attached).
The course book is Esterby and Thrope book and the focus is chapter 10 (multivariate analysis). There is an example 10.5 that I think is relevant to this question and can be used to asses the validity of their claim, but Im not sure.