Just as flex and derate is interchanged willy nilly these days, as if it the same thing, so is autothrottle and autothrust.
It is not the same thing.
So whats the difference then?
Flex - reduces thrust a known amount
Derate - reduces thrust a known amount
Autothrottle - automatically changes the engine output based on certain limitations
Autothrust - automatically changes the engine output based on certain limitations