I don't think I believe in the distinction. "Lack of discipline" is a good descriptive synonym of not being able to focus on unpleasant tasks, yes, but as a way of pointing out a perceived flaw of character, it doesn't feel very useful. After all, a person that "lacks proper discipline" isn't usually able to fix it themselves. You can't "just get" disciplined.

Also having the capacity to concentrate but refraining from using it is an active choice; people doing that won't be complaining that they can't focus.