They did get advice from some of the most experience startup investors in the world. Who's biggest question was why they hadn't started charging users. Taking advice from highly experienced people is probably better then trying to do your own research.
How would you even research such a decision if not ask very experience startup founders and investors?
Imagine if FB had a premium plan in 2008, they wouldn’t have made it.
In this case it probably makes sense to charge early on, but it doesn’t look like they did much research on the decision