I think those numbers are a bit too pessimistic, but completion does vary quite a bit from kit to kit and certainly for kits vs. plans. For instance, if you call everyone who buys a set of plans a kit builder, you start to get some pretty low completion numbers, but I would maintain that many, maybe the majority of, plan buyers never even start the construction process. Are they builders? I would say no.
GlaStar kit builders, with whom I have a lot of experience, tend to finish their planes at a fairly good rate -- almost 400 completed vs. around 950 kits sold. Some took 10 years or more, but a lot of them finished. Of these I would say that something like 75% were finished by the original builder.
My guess is that other well supported kits like RV series, Lancair, Glasair Sportsman, and RANS series get finished at a pretty high rate, and more often than not by the original builder.
On the other hand, I think that many planes that do not come as kits and especially those that do not have a large following, prove to be too difficult and/or time consuming for most builders and do end up collecting dust long before they are ever finished.
Good planes that perform well and have broad appeal that can be built from good, complete, well documented kits, with good builder support groups lead to finished projects.