You can add “feed/” to your site’s URL in your browser and the XML feed data will be returned for your examination. I just did this for you. The post content has successfully been suppressed, but not the excerpt (referred to as the <description> in XML). I don’t know why adding the empty string filter callback is not working, it works correctly on my site.
It’s conceivable your theme or plugin is also filtering the_excerpt_rss and overriding our empty string code. We can try having our callback execute later so maybe it’ll have the final say. Modify the_excerpt_rss code like this:
add_filter('the_excerpt_rss', '__return_empty_string', PHP_INT_MAX );
If that still has no effect, try flushing any caching that may be in effect. If still no joy, the cause is likely due to your theme or a plugin. Determine which one is responsible. Selectively swap themes (adding our filter code to the new theme’s functions.php) and deactivate plugins until our filter code starts working as expected.