Plasma's power management has no way of knowing which processes are important enough to block the computer from going to sleep. Silently disabling energy saving and screen locking for any process that exceeds some arbitrary CPU usage threshold would be a pretty bad thing, as it would drain the battery and give other people access to the computer without the user knowing. It is the responsibility of the individual applications to request power management inhibition where it makes sense, and Plasma's power management does support picking up on those requests. If your video playing program does not do this, please report it as a bug to that application.