It's a nice way to make the reader think 15 when the reality is 8.

'over' can also be used to indicate something took place throughout the whole timespan. Short form of 'over the course of little less than a decade'.

Presumably it should have been "over nearly a decade".

So "a decade" :)

No, "throughout nearly a decade", as allannienhuis says.

https://news.ycombinator.com/item?id=11520368

Nearly a decade, e.g., "9 years".

Then "over" makes no sense, right?

or it could mean exactly a decade!

It means a writer is trying too hard.