The technical ISO definition of usability indeed relates to the user’s (or users’) effectiveness, efficiency and satisfaction in relation to specified goal(s). I was aiming to use plain English terms — I would describe the above as ‘ease of use’.
Since circa the early 2000s, leaders in the field of usability/HCI such as Donald Norman conceded that analysing ‘effectiveness, efficiency and satisfaction’ wasn’t enough. We need to look at the more holistic ‘user experience’ — e.g. what is the emotional state of the user when engaging with the software?
It may be effective, and the user may feel satisfied that they have achieved their goal (good usability), but, beyond that, what feelings does the software evoke in the user? Does it for example create aesthetic pleasure, the way they feel when they see a beautiful picture (good user experience … assuming the product is also usable)?
That is why I argue that usability is in fact a subset of user experience.