Firstly, it is common to use hyperbole when describing importance of a practice or the greatness of a special place.

But what you have quoted is not much of a hyperbole. Swami Shivananda is not repeating from older texts, rather he is using them as an well accepted source. Talking about one's own acheivements is somewhat of a taboo. There are many people who have acheived mantra siddhi, and one can potentially also meet them. The difficult part is putting in the dedication to the necessary amount of sadhana.

Note that it is not possible to most of the guidelines in the book, but nevertheless it is good to read and find out what is considered beneficial. Also, certain mantras like pure 'Om' are only meant for sanyasis as they interfere with regular life.