(As caveat, I can’t find my copy of 9241, so I’m going from memory here.)
If you take the broadest interpretation of task suitability, then, yes, the other guidelines are subsets under task suitability. However, task suitability is more than the sum of the remaining guidelines. A UI can require little learning, be well-customizable, conform to user expectations, be self-descriptive, controllable, and prevent/tolerate user errors, yet still make completing the task slow, arduous, and tedious, and yield an unsatisfactory result.
Imagine, for example, a wizard UI with 43 screens, each with a couple paragraphs of highly detailed perfectly understandable documentation to help the user set a control. User completes the task without error, but the UI would be entirely unsuitable for a task that requires timely completion (which is most tasks). Or consider a super-automated system whose entire UI is a single button labeled “Do it.” Hard for the user to make a mistake, but if the automation isn’t particularly effective at guessing the users’ parameters for the task, it’ll be unsuitable.
If you take such a broad interpretation of task suitability, then you’ve basically making that guideline say, “A usable UI is usable,” which isn’t particularly helpful for guiding design. To be helpful for guiding design, take a narrow interpretation of task suitability focusing on the sliver that is not overlapped with the other guidelines. Look at your design and imagine the “normative” interaction. That is, imagine the users interacting without error –they’ve learned everything they need to know beforehand, they’ve appropriately customized the UI, everything goes as they expect, they understand exactly what the product is doing and everything expected of them, and they freely choose the best way available to interact.
Now, did the users get a result that meets all the goals of the task? Did they do so quickly without a lot physical or mental effort? Or does normative interaction have a lot of clicks, scrolling, reading, scanning, cross-referencing, or mental manipulations that can be eliminated? Does it make simplifying assumptions about the user’s goals that are not always valid? Applying the guideline this way can give you specific ideas for improving the design.
