That’s correct. Zone X boundary to destination tickets are available to season ticket holders. Not widely advertised. They allow you to travel on fast trains that don’t have to stop at a zone X station. And if you have a Gold card, you also get a third off.
I used them all the time for trips to Reading and Brighton. If I wasn’t lunching in Angel, could have told you straight away. I may have spent an evening reading all the t&cs.
Read this for a summary of Gold Card benefits.
http://diamondgeezer.blogspot.co.uk/2014/11/gold-card-upgrade.html?m=1