The $query variable is passed by reference, so it isn’t a copy of the $query variable inside WP_Query::get_posts(), it’s a link directly to it. Any changes to $query inside the callback function will be applied to the variable in the calling function.

Your show_posts values end up being passed onto posts_per_page anyway…(because it replaces the older showposts).

http://codex.wordpress.org/Function_Reference/query_posts#Post_.26_Page_Parameters

A quick and dirty workaround is to add extra conditions to the function that is doing the pre_get_posts filtering to make sure it’s really working with posts or pages.


function filter_homepage_posts($query) {
if ( $query->post_type == 'page' || $query->post_type == 'post' ) {
$limit_number_of_posts = 5;
$featured_category_id = get_cat_id('Reviews'); // by cat name...
if ($query->is_home) {
$query->set('cat', $featured_category_id);
$query->set('showposts', $limit_number_of_posts);
}
}
return $query;
}