"Dialect" here means mutually intelligible. Mutual intelligibility is a fuzzy threshold, incidentally. I was in Spain once hanging out with an Italian guy. The Spanish speakers around could make sense of his Italian but he couldn't make sense of their Spanish. I think the essential problem was that they were more interested in understanding him than he was in understanding them, so they put in more effort.
Hypothesis: The Italian guy might have a better chance of picking up Spanish if the Spaniards were to speak a bit slower than usual :) Especially at the fuzzy mutual intelligibility boundary, a lower communication speed helps.
