100 million reviews/year is only 3 reviews per second on average. Sure, they seem to do more then just that, like voting, comments, etc. But it still seems like something an old school stack could handle on a single large instance.
Reading between the lines it seems the problem wasn't scaling, but programmer productivity. Smaller code bases is often easier to work with so I guess they solved that by dividing it up into many small services. The blog could use a more detailed description of the problem they are actually solving.