The initial version of this feature only stops HTML5 audio/video preload. The “disable auto-play” feature was not incorporated due to a Netflix interoperability issue; disabling auto play is anticipated to be in an upcoming release - that's from the development team since I had requested this feature.
You can check to see whether the "Audio/Video data saver mode" is on under Settings, Advanced, Privacy and Security.
Just remember preload and auto play are two separate things you can have preload without autoplay, but not vice versa. Probably some add-on or extension is available for Chrome that will work for disabling auto play. Most disable autoplay add-ons don't handle the impacts of preload without auto play (for the rest of the world it's an annoyance not a data issue).
Note it doesn't save you from the abuses of preloading caused by java script nor can it. Similarly abusive java script data usage is not limited to simply video/audio. But Chromium (upon which Sparrow is based) does allow turning off java script on a site basis - which I use on several sites that use java script solely for abusive purposes, automatic reloading/refreshing, etc.
With disable auto play and a complete implementation later it'll be invaluable to satellite internet users.
The only other thing I could ask for out of the development team would be an easier way to just declare a site encountered off limits to java script with a single click (similar to YesScript). Kind of clumsy the way it's implemented within the Chromium base but workable.