Perhaps this is a naive (or dumb) question, but what will all those new engineers do? Reddit is doing what it has always done why does just scaling up require so many more engineers?
The preverbial wheel of scalling solutions needs to be re-invented, yet again. Not all services are created equal so no one is truly re-inventing anything, just applying unique solutions to unique problems. That takes manpower.
Heck just to move from Amazon to a dedicated will take a team larger than they had a month ago. That's right from the horses mouth. Don't have the energy to fish out the link, though.
Note: Please don't edit your comment to make me look foolish. :)