The Brit in that video speaks Japanese fluently, so it's easy for native speakers to understand him. He does have an English accent, but it doesn't impede with his ability to be understood (after all, even native speakers have accents).
If you're bad at speaking a language, then people are going to have a hard time understanding you -- doesn't matter if it's English, Japanese, French, German, Latin or Klingon.So yeah, a person who speaks terrible Japanese would be just as unintelligible to Japonophones as a terrible English speaker would be to Anglophones.
