Derated and Flex are two different things!
Derated means less than max TO thrust. I don't know Boeing but it seems a synonym for reduced TO thrust. Other manufactor use the term derating for reducing the thrust for a given type, i.e. they reduce the max possible thrust for a certain engine on their airframe. If they do, you cannot go above that or you can override it by "firewall thrust", depending on type.
Flex is a derating method. It's based on assumed temperature. In the end, it's also reduced TO thrust. That's correct.
Dani