The thing that struck me, reading that, is how much time he could have saved if he'd memorized fewer words and more rules. He had to suss out the grammatical relationships between all the words derived from the root for 'work' himself, whereas if he'd learned the paradigm first, he could have anticipated the derived forms without needing to hear them. And he went to all that trouble devising a mental image to remember 'motele,' which looks like it's got to be borrowed from 'motor' into a language with no l/r distinction-- learning a few basic phonological rules and knowing which languages Lingala borrowed the most from would probably be enough to understand a huge swath of the vocabulary, since Lingala is a trade language and most of its vocabulary is probably borrowings and Wanderwörter.

tl;dr-- learning != memorization. Language learning really, really != memorization.