For example, "We're competing for attention with teenagers who would rather be playing Angry Birds," or "You need to explain this in a way that your grandmother who thinks the internet works by magic would understand."
What these really mean is, "We're competing for attention with teenagers who would rather be playing on their mobile phones or doing something else", and "You need to explain this in a way that anyone would understand, even if they don't understand how the internet works at all."
The speaker has no idea whether the other person's grandmother understands the internet or is even alive. But they use that phrase in an attempt to be slightly humorous. (The examples above aren't very funny, of course, but they might make people titter if they are said in the right way.)
I thought this might be synecdoche, or one of those other Greek terms, but it's not. I am wondering whether there is a term for it?
Edited to add:
I think it is synecdoche after all, and the best way to describe this exact type of construction is "facetious synecdoche".
(I should have made it clearer that what I wanted was a noun or noun phrase that describes this exact type of construction. I didn't want a word or phrase that describes this type of construction and lots of other ones as well, such as just "facetious".)