I think the point is that if it fits on a single drive, you can still get away with a much simpler solution (like a traditional SQL database) than any kind of "big data" stack.
I always heard it as "if the database index fits in the RAM of a single machine, it's not big data". The reason being that this makes random access fast. You always know where a piece of data is.
Once the index is too big to have in one place, thing get more complicated.
