It takes a few weeks of practice to be able to intelligbly understand someone from India, Kenya, England, the US, or Australia, if you're from one of the others. Cadences and idioms are all off.
We often modify our speech for the audience - put someone in a party situation where they are the odd one out and it will be harder for them to follow along.
I've seen a fair amount of British TV, and I'm fairly comfortable with the idioms and vocabulary differences. Less so than American TV, but I think that's fair, since I'm not immersed in the culture.
I've seen much less Australian TV, but a lot of the slang was utterly unfamiliar to me, in what I have seen. I'm sure that if I watched more, it would become more familiar.
I don't understand what is so controversial about the idea that other places use the language slightly differently, and it can take some time to become accustomed to the differences.
Nothing. What's controversial is equating "other places use the language slightly differently, and it can take some time to become accustomed to the differences" with "it takes a few weeks of practice to intelligibly [sic] understand someone from another place".
The first of those, yours, is a very weak claim. The second is a very strong one.
I said "this is dramatically overstated" and you're trying to contradict me by saying that a much weaker version of the claim is true. Yes, it is, that's what "overstated" means.
My best friend is Australian, the idioms are often nonsensical or require a bit of thinking due to being a variant of an idiom I'm familiar with. e.g "a roo loose in the top paddock"