Pete,
That book repeats a bunch of papers that essentially look at population data sets, if any. It was published in 2010, meaning written in 2009 at the latest, and not up to date on issues like SVC vs. FVC in ALS. One of the papers it quotes most heavily was published in 1999.
I haven't seen those particular numbers to be thresholds in real life for the end of life. For example, we have many people here who have lived years with FVC<30%. My husband was one of them. I can tell you that the literature on his other rare disease, Marfan syndrome, is equally lame and applied to few people on the Marfan list of which I am a member.
Physicians like to reduce uncertainty for themselves, and this book purports to do that, but it's a feel-good for docs, not written for you. Syndromes are like that.
Best,
Laurie