Do one thing well does not really apply to larger user-interface applications that inherently have to aggregate lots of unrelated functionality. It applies to one-shot commands that you'd use in shell pipelines; that's why the other commandment is to use text streams. The point is that by combining these small utilities which do one thing well, you can create the aggregated functionality you need to perform a higher level task. Whereas if you didn't have these little utilities which do one thing well, you'd be scratching your head trying to figure out how to do something that a big monolithic application can do for you but which you need in a different context, i.e. where running that big monolithic application to do that one thing is very very incovenient if not impossible. Why do people quote this out of context?
If you stop and think about it, it's really pretty much impossible to define the "one thing" a code editor does.
Forcing the user build everything out of shell pipelines is not the goal, and achieves nothing. At some point, it is appropriate to build the aggregate and combine these functionalities for the user so that he doesn't have to do it himself. And in many cases, you end up reimplementing the thing because it is more efficient that way. The shell utility is still there for the user should he need it.
If you stop and think about it, it's really pretty much impossible to define the "one thing" a code editor does.
Forcing the user build everything out of shell pipelines is not the goal, and achieves nothing. At some point, it is appropriate to build the aggregate and combine these functionalities for the user so that he doesn't have to do it himself. And in many cases, you end up reimplementing the thing because it is more efficient that way. The shell utility is still there for the user should he need it.