Well, except for iaido and batto.
Weren't there a number of minor deities with him during those "solo training" moments, anyway?
Well, it's not canonical, of course, but there's no denying that his weapons training was unusual, or that (as he himself said) it was done with a different type of intent.
The deity thing is really a longer discussion - but suffice it to say that the point holds, he spent hours training alone every day. If he
did it, why is it impossible for anyone else?