I did it! Jesus Christ, took me more than 4 hours to find that one line. It’s in activity-loop.php. Now, that was the easy part.
<?php if ( bp_has_activities( 'action=activity_update' ) ) : ?>
that’s how it’s supposed to look if you want to hard code the filter to one specific value. The hard part was finding out the proper argument for the bp_has_activities function. Well, now I got it. But seriously, why are you making it so hard for users to modify?