Tanker, I can't speak for why the author "had to" dip the brush in thinner before the paint, but, I can tell you why I have always done it with any model paint I have ever used (since about 5 years into my modeling time)
I do it because it is the way brush painting is done, you dip the brush into your matching thinner, then blot some off or don't blot it,,,,,,and then lay the brush into your color in or on your palette, thus loading the brush for a few strokes on the model,,,,,,the thinner sort of "chases" the paint off the end of the brush onto your work surface, clean up is far easier afterwards, also
it is the same method that painters use when working with brush paints,,,,,,,
the method of just dipping the brush into a paint jar and grabbing a glob of paint, tapping/or scraping that on the rim and scrubbing it all off onto a model is a "plastic model newbie" method, and is the #1 thing at the root of the "I can't brush paint well" complaints,,,,,,that method also ruins countless brushes, by getting paint or thinners up into the ferrule that holds the bristles, weakening those, and having those bristles then fall out of the brush