This is rather philosophical:
You draw an opera in your map. By doing this, you acknowledge that in the OGF world the musical genre of "opera" exists. But this genre only came to be because of European music history of many centuries, so you acknowledge that this has taken place, too. So there has to be Europe, which there isn't in OGF.
If OGF is a different world, why would they even have operas? Would they have orchestras? Wouldn't they be playing totally different instruments?
Another example: The OGF encyclopedia states "Avoid referencing real world companies, organizations, and famous people or characters". Alright, no Coca Cola etc. But what about math and science, for instance? Do we have to come up with a new name for Euler's number just for the sake of it?
Of course all of this doen't really matter much for the mapping itself, but I can't wrap my head around the concept that the OGF world has to be similar to the real world when no real world history, people and places are supposed to exist...