Even after reading it over 50 times, I sometimes discover a point that I have overlooked. I have explained to my 4U students that the Cambridge 4U is not an easy book to follow. Even for me - after my 10th reading I find something that has escaped me. One way of explaining the difficulty is: this book would have been an excellent and easy-to-follow text were it 3 times as thick!
The book has been distilled to the bare essentials - hardly any user-friendly explanations. Each paragraph could do with another 2 or 3 paragraphs explaining the underlying concepts more fully. Each sentence is pregnant with facts. Not an additional word conceded by way of explanation. I have a copy of the American text: University Physics (Young & Freedman; originally Sears & Zemansky) that has 1500 pages of fine prints; were this done in the Arnold & Arnold style, it'd be no more than 500 pages. This textbook is so comprehensive and explains things in a very clear and helpful manner. The illustrations, like many American texts, are just outstanding. The Cambridge text is terse, written in the spirit of texts prior to the 1960s. Keeping the text slim may also have been a priority, to keep the cost of publication down.
To answer the question: Yes, Cambridge Extension 2 is a very hard textbook for self study for the vast majority.
Because it is a widely-adopted text for 4U, I suspect no small number of Maths Ext 2 students have been driven to seek the help of coaching centres and private tutors.