I think the level and the position of that book is embarrassing. It covers phd level materials but not presenting them at that level. It is called "theory and method" but is not doing well on both aspects.
I guess when one is studying at phd level, one should have already get used to (or getting used to) definition-theorem-proof style. In particular, by definition of econmetrics, it should be the most math-heavy compared to other classes like micro and macro.
The first challenge you have to face in graduate level econometrics is interpreting probability in measure-theoretical sense, and asymptotics. This book does not introduce probability deeper than your average intermediate/introduction undergraduate statistics classes. It introduces LLN and CLT, but never differentiate between the different modes of convergence and the various LLNs and CLTs. It does not even formally state anyone of the LLNs/CLTs and under what conditions they hold. As a result, when talking about convergence there are A LOT of handwavings. These things are crucial in graduate econometrics classes in my opinion. Worse still, most theorems and definitions in this book are not separately stated, they are buried in the text. This makes it very inconvenient to use as a reference.
But the book is by no mean a "very easy" book. As I said above, they are just not using the right language or style to present the materials. I read the first half of the book in my final undergraduate year, the result is that I was very unsatisfied with it. Very often I turned to their older book "estimation and inference in econometrics", which was much better written. In the preface of this book "econometric theory and methods", they said that they wrote this new book thinking the older one was in some sense too advanced for first year graduate econometrics. I strongly disagree with that.
The new one is not rigorous for first year phd program. I must admit that their chapter on geometry of OLS is quite good. Understanding the geometry there saves you a lot of algebra. But this is covered in their older book too. Besides, you don't see OLS that much in graduate courses these days.