Is there a reason you used the weight paint instead of assigning the outside vertexes to the thickness group at a lower weight?
It sounds like you're asking why paint vertex weights as opposed to manually assigning vertex weight values to selected verts. If that's the question, it was probably just my preference at the time. Weight painting is a more artistic approach which is often what I prefer to more rigid, technical approaches.
Watching back over that section, I think it would be fine to manually assign vertex weight values if you prefer.
Ooops one more thing - By artistically painting the vertex weights, it results in an inconsistent thickness along the tentacle. I think that results lends itself to the organic nature of the tentacle, but perhaps it's a scientific fact that tentacle thickness is consistent ¯\_(ツ)_/¯