I took a look at Chapter 1 of Mankind's Search for God, specifically paragraphs 11 and 15 like you mentioned. I completely see why those lines about studying other religions being 'educational' and 'mind-broadening' jumped out at you!
However, if you look at the context of who the book is actually talking to, it changes the meaning completely.
This book was written as an outreach tool for the public, especially for people who were born into other religions (like Hinduism, Islam, or Catholicism) and are afraid to look at anything else.
Notice how paragraph 7 talks about us in the third person, calling us 'these people.' The writer is talking to a non-Witness reader, trying to lower their defensive walls so they will be willing to learn about other histories.
The chapter isn't telling an established Witness to go out and study other faiths. In fact, if you look at the conclusion of the chapter in paragraph 25, it brings the reader to the real point: 'surely the question should be, Is my religion good enough for the creator... This presents a challenge to each person—to examine the evidence and prove for oneself what the acceptable will of God is.'
The very next paragraph points to the Bible as the only standard. So the book is really telling a non-Witness: 'Don't be afraid to examine religious history so you can see that your birth-religion might not be the right one, and then use the Bible to find the true one.' It's an introductory bridge for the public, not an instruction manual for Witnesses to study other doctrines."
This keeps the tone objective, uses their own source material to clarify the point, and gently shows him the difference between an outreach strategy and internal instruction.
Bottom line: We have the truth! We don't need to study other religions to be sure. We already studied and saw from the Bible that we have the truth! In fact, look at chapter 15 of that same book.
https://wol.jw.org/en/wol/d/r1/lp-e/1101990047