It's suggesting, I think, that it is at its maximum speed when it is 'in full charge', i.e.charging as hard as it can - of a horse and a lot of other animals we say 'flat out', ('belly to the ground' is the equivalent French expression, I think). So the answer to your question seems to be yes on both counts.
I think the author mentions charge specifically, because that is likely the time that maximum speed is reached. I don't think rhinos attain maximum speed for any other reason (probably not too many natural predators). The use of 'in full charge' then is not only indicating that this is the maximum speed, but also the conditions whereby it would be reached. It also stresses the fact that an animal that appears to be a lumbering hulk can go quite quick if he wants to trample you