I think we're not interpreting the original comment in the same way.

In most places, I think, when driving on the highway, flashing your lights when behind someone means basically 'I would like to overtake you'. Same here in the UK. But that's very specific to that context. You would never see a 'go ahead' context that would mean 'get out of my way', right?

But what the original comment means is there are some countries where you'd think it was 'go ahead' but it really means 'get out of the way'. Like if you're both on a main road, and you are signaling to turn into a side road, the opposing car flashes the lights and that means you can turn. I assume the same in Serbia.

But in some places that can actually mean don't turn, I'm going first. Which I think is what the parent is describing.

You are right that I did not read it the same way, and yes, the unwritten rules are matching in Serbia. FWIW, I've mostly switched to using left-turn signal to indicate "I'd like to overtake", which I've seen done on EU highways.