That possible explanation has been put out repeatedly.
Given how successful exit polling was until 2000 in the USA, and given how successful it is in elections around the world, I do not believe that this is a plausible explanation of recent discrepancies.
But, as is common in this sort of situation, I have no way to prove it.
Nate Silver wrote a great article in 2010 about why the notion of exit polling itself has a lot of fundamental problems[1]-- among them, the fact that it is extraordinarily difficult to get an truly random sample.
If you read the article, there are several instances of the same problems causing very large errors in elections prior to 2000, for example, several miscalled states for Clinton in previous elections. To claim that these polls "suddenly" stopped working is demonstrably false.
Given how successful exit polling was until 2000 in the USA, and given how successful it is in elections around the world, I do not believe that this is a plausible explanation of recent discrepancies.
But, as is common in this sort of situation, I have no way to prove it.