Confirm above. Have used it myself last oct.-and next week (both on a 50% discount pass)-last oct right after arr. at Barajas airpt at the tiny office they have there (no Qs, apparently a kind of apprentice who had to fone on how to do it-but when that was done all went smooth for all trips needed the upcoming weekend).
Major stumbleblocks (and yes-in busy times in ESpana trains do sell out, sometimes days before) are all trains to/fro MADrid at tipical weekend timings and the very long-dist AVE bypassing MAD (like Bcn-Malaga etc.), problems also tend to be much higher in main summer and also when its busy in ESpana: Easterweek.
Your route mentioned is not covered by them-but now that the 2 other low-cost competitors run some main lines on hi-speed, Iriyo and Oui-GO, it may be worthwhile to check if their fare is lower as passday/cost +cost for REServacion on RENFE.
In all frankness: IF no curbing measures would be taken, all seats, or nearly all, would be taken by passholders in the main summer season.