When Tom and Gareth built the original version of this at History Hackday in London they mentioned it in their presentation (it was being filmed so I presume it's online somewhere?) - it's based on the English Wikipedia data so it's skewed to a English speaking world view.

Although even more generally European and Asian history (as academic fields) have been much more deeply studied than African history.

I guess that's 'cause the West has the most geotagged Wikipedia articles with time attached...

That's exactly what I was thinking. This is more of an interesting commentary on Wikipedia articles than any kind of accurate view of world history.