But on the coasts it's about (but not quite) as complex as London might be. New York may be the king of all accent centers in the U.S. (followed maybe by D.C. or L.A.) You get all the various New York accents (not unlike London's various native accents), various native ethnic accents (like AAVE, which even has accents within), regional accents, the various north East Coast accents (every city from Boston to D.C. all have different native accents) as well as rural accents, you also get all the various local accents from the U.S. and Canada since everybody ends up in NYC, then you get all the foreign accents (at various levels of English attainment) from immigrants and tourists and finally you also get all the various native-English accents, including many (but not all) of the ones from the UK.
To be fair, the distance between native accents in the U.S. is generally not quite as far as between native accents in the U.K. I've been in the U.K. and Ireland a few times and there really were places I ended up where the local accent was not intelligible to me, and I struggled at times in London. I also notice generally wider distance between the various accents in London that I assumed were native London accents than I've experienced anywhere in a single place in the U.S.
If you played several samples to the audience, they'd start to recognise more words.
Actually, I think she influences the outcome by telling her audience the spoken sentence has been "transformed into gibberish". By doing this she has planted the expectation to not comprehend the sound, since "gibberish" by nature cannot be understood.