I think the point he was making was that the company in question never tried to scale horizontally with MongoDB because, in their words, "we believe horizontally scaling shouldn’t be necessary for the relatively small amount of ops per second we were sending to MongoDB."
Yet, they went and scaled horizontally with Riak and experienced pain.
Their opinion that it did not make sense to have to horizontally scale the "relatively small" number of ops they were sending to MongoDB is certainly their own, but then they horizontally scaled with Riak anyway and boasted about their 25MM ops per day scaling ... which, averaged out, is only about 280 ops per second.
In short, it was far from an apples to apples comparison.
