I would argue that an "arc" fit two criteria:
A) Be more than one chapter in length
B) Exist an independent story; complete with a start, middle, and finish; able to be to read semi-independently from other story-lines, and self-contained
I would argue that the "King Ton" story-line is a part of an overall arc, much larger than the individual story, whereas the "Roots Arc" story-line is self-contained and complete, without a connection to a large plot-arc or greater story-line. So I would class it as an arc, especially in regards to classification within the wiki and/or fandom, as it would make it more difficult overall to place, as then which other arc would you ascribe it?
Feel free to disagree, though.