Re: Is it an art within itself?
Ki Society teaches a breathing exercise--I don't know if this is what your sensei was referring to--called "ki breathing".
It's certainly possible to practice and study this for years before doing it really well. When I was visiting Maui Ki Society I discovered just how far out of my depth I was--ability to sustain long, slow, calm breaths, ability to keep centered while doing so (sensei would test us by surprise), abiilty to sit seiza for a long time, ability to keep the entire body relaxed while slowly suffocating.... It was one of the hardest things I've yet tried in aikido.
Kashiwaya sensei has written an introduction to ki breathing which is easy to find on the Web, and the Maui Ki Society
handbook also has some stuff on it, if I recall correctly.
If your sensei means, more generally, controlling and using your breath while doing aikido--I can well believe that mastering that will take the rest of our lives. I'm still working on the basics, such as breathing out when thrown so that I don't hurt myself on landing. I suspect each throw has a particular breathing pattern that will work best for it; I think my teachers aren't talking about that, not because it isn't true, but because the big physical mistakes have to be corrected first.