Are you sure he meant dialect and variants , not accent ?
We Asians, speak in a different accent than all of those in Europe, America, etc, and it can sometimes be difficult for some native English speakers to understand it, since its not commonly spoken.
I am a 16 year old Pakistani teen, a country not too far away from Singapore, interacting with at least a couple of 50 to 60 years old people here on MFK, from all over the world, without having any trouble understanding their each and every word.
Sure that English is evolving at a astonishing rate but the use of social media, internet, etc has made it very easy to keep up with things.
So in my opinion, that professor has hit his head really hard against something solid.
Its not just the accent, its speech patterns, local slang , figures of speech and the age of the person speaking that can make a conversation very hard to follow.
As
esoxlucius
said its easy if its written down but listening to it and speaking it are very different. When i was travelling in the UK it wasnt just the regional accents that were the issue it was the words they used and the way they used them.
You would be shocked how many different words they use for sandwich depending on what part of england you are in, it can be called a bap, a cob, a buttie, a sarnie and sometimes just a sandwich. Here in australia we call them sangas. Imagine you are in a country with what is to you a very hard to decipher accent then someone asks if you want a sandwich from the shop for lunch but the words for lunch, shop and sandwich are all different to what you use.
It would take you at least a few seconds of translating in your head to be able to reply.
If you were working on a building site in australia as i was when i was an apprentice you would hear something like "hey mate its smoko, im going to the servo, do you want me to grab you a sanga?, my shout, i owe you a few bucks from the schooners yesterday arvo. I might get some darts too"
Imagine the accent and noise of a building site in the background too, Can you translate that?