This disclaimer on page viii sums it up:
We hope this chronology will make it easier for Star Trek writers to remain consistant with what's been established to date and for fans to keep track of Star Trek's elaborate back story. We do not, however, want this to intimidate our writer friends or to inhibit the imaginations of fans who may have differing interpretations of the Star Trek timeline. As such, we encourage both fans and writers to take this material with a grain of salt and to enjoy it in the spirit it was intended, as a fun way to explore the Star Trek universe.
As First Contact
and Star Trek: Enterprise
showed, writers were more than happy to rewrite the chronology and disregard the conjectural dates within. It's still a fun book and an excellent resource.