Is something that is ‘simple’ the same as something that is ‘usable’?

Short answer: No. However, one has sometimes been over simplified to mean the other.

Here is an example:

There isn’t a chance in hell that I would let anyone perform brain surgery on me using an interface that was hassle-free, foolproof and straightforward so much so that any member of the public could do it. However, I would let a qualified brain surgeon to do it using an interface that presented the information that he or she needed practically to afford them to do their job competently, efficiently and effectively.

This is the difference between creating something that is easy to use and something that is usable and therefore fit for purpose.