We don’t understand what Gravity really is, so we cannot make assumptions that interstellar space travelers will have to deal with acceleration-induced gravity issues.
For all we know there may be a way to create a “gravity shield” that blocks gravity from the inside of a ship, or creates an artificial gravity field that counteracts acceleration-induced gravity.
The author is distinctly not talking about Newtonian gravity, and he states that early on. He is using the term “g” to compare inertia to gravity. In general relativity the perceived forces of gravity and inertial are equivalent.
Ersnt Mach (yes, that Mach) said that when the street car lurches, it is the stars, not street car that makes you fall. Or as John Wheeler said, “Spacetime tells matter how to move; matter tells spacetime how to curve.” Inertial effects are result of mass. If there were mass that would be no space, so it is meaningless to speak inertia in an empty universe, as there is no such thing.