So right now, that code is doing this:
If we’re not in category 8, show the regular excerpt.
If we’re in category 8, sho the regular excerpt for the first post, and no excerpt for the others.
So from what I can see, those functions aren’t removing the excerpt from a specific category.
If that’s what you want to do, you need to do this:
add_filter( 'excerpt_length', function( $length ) {
if ( is_category( 10 ) ) {
return 0;
}
return $length;
}, 1000 );
add_filter( 'excerpt_more', function( $more ) {
if ( is_category( 10 ) ) {
return '';
}
return $more;
}, 1000 );