Not entirely correct. We opt out of the system level feature because it is meant for telephone style calling and hence media players etc. react to it by pausing (which we obviously don't want).
We do "manually" attenuate applicatications and could, technically, attenuate single applications. At this point however there is no such feature and we have no plans to implement something like that. If you think you can make a compelling case for it feel free to enter a feature request into our tracker. Be aware however that our development resources are pretty restricted so it might take a while (if we decide to implement it).