Why vertices of directed acyclic graph are considered strongly-connected components?
This has to do with the definition of Strongly Connected Component in graph theory.


  A graph to be said to be Strongly Connected if every vertex is
  reachable from every other vertex.


The smallest possible graph of any type consists of a single vertex.
Since that vertex can reach itself (since it is itself), the graph therefore meets the criteria, and can be considered Strongly Connected.

In this image, the nodes at the bottom right, and the top middle are examples of smallest possible strongly connected components: