Roughly 2000 words are enough for a basic level. Because some of these languages are related, their vocabulary and grammatical patterns overlap.
You are underestimating the capability of some people to memorize certain information. Sometimes it is called "eidetic" memory. The best chess players just don't forget any chess position they ever see. Similar reports in music. Most of them are useless when memorizing any other form of "data". The carpet cleaner in question probably has that facility for words, phonemes and grammar. He just doesn't forget that stuff.
You are underestimating the capability of some people to memorize certain information. Sometimes it is called "eidetic" memory. The best chess players just don't forget any chess position they ever see. Similar reports in music. Most of them are useless when memorizing any other form of "data". The carpet cleaner in question probably has that facility for words, phonemes and grammar. He just doesn't forget that stuff.