That's true, but look at the context of jay's content. Newbish to say the least...the question posed seemed to me like he wanted some kind of muscle development, and due to his newbness, I assumed that this would be hypertrophy related.
I could be wrong, but I doubt it.
Still, the primary function of the speed bag is not to develop your physique. Despite the fact that this happens to an extent. It will work your arms and whatnot, but only to the point where you've adapted to it.
And, technically speaking, the speedbags will work the minute muscles that contract the iris, hearing, etc...