The mixer brush referred to has particular qualities; allowing sampling and replicating parts of the canvas, blending that with palette colours, and smudging.
You often see it manifest in digital paintings as areas of texture repeated as a stroke. That is similar to using the clone brush in Krita, but has a lot more flexibility as mentioned above.
Note: this is only my understanding from what I’ve seen and read - I’ve never actually used it myself.
Here’s the thread from a few months ago where we discussed expanding the clone brush functionality: