In my mind, physically being able to hold a lens and take a sharp photo doesn't make a lens a handheld lens. I mean, by that definition, even very large and heavy lenses could be "handheld", because, yes, if you really wanted to, you could do what it takes to hold it and take a shot without it getting a blurry image.I like to consider myself normal, and I could hand hold the 150-600mm S but the C is so much more comfortable. With all of these things, some can manage to hand hold a particular lens, others find it too much of a strain. Some can hand hold a 600mm II. Others find a 100-400mm II as much as they can take. In general, it is better to keep lenses light.
Handheld lenses, to me, need to have a mounted weight, balance, and size that is comfortable for someone who engages in a lot of photography with heavier lenses to shoot as a handheld shooter for many hours (at least, the length of an event) without fatigue. I cannot imagine using the 150-600S for the duration of even a two hour event (never mind a half-day sports event) handheld and not having fatigue hit pretty early - I'd be lowering the camera to give my arm a rest frequently, probably missing moments, and that to me makes it non-handheld.