The book seems to be angling more towards the impact of the Bell System and Bell Labs, which could certainly be argued as more impactful than the telegram, even if downstream. AT&T and Verizon both emerged from the Bell Breakup. Western Union still exists, but as a shadow of its Guilded Age self.
Telegraph seems like the more significant milestone, but putting that in the title of a book wouldn't sell many copies.
Which, after all, is why people want to read about it. Most people don’t have much firsthand experience with the telegraph.