It only states what is mandatory to register because that how laws work. If you tried to make a list of exceptions it could be a mile long.
Bicycles do not have to be registered. Nor do electric bicycles, with a max speed of less than 45kph
Anyway, I will happily ride my electric bicycle, max speed 40kph, until someone can find an official site with other information. So far I've been stopped hundreds of times driving a car or motorbike, but never been fined for anything. Never been stopped on my electric bicycle after 8 years of driving around daily.