This:
apply_filters( 'image_downsize', false, $id, $size )
doesn’t call the image_downsize
function, it applies any filters hooked to the image_downsize
tag, which would be a different function.
The only way that would cause recursion is if the filter you hook in turn called that function:
add_filter( 'image_downsize', 'wpd_downsize', 20, 3 );
function wpd_downsize( $return, $id, $size ){
// collapse the universe
return image_downsize( $id, $size );
}