Custom Post type and Custom Field WP_Query

Currently your meta query is exclusive and you need to make it inclusive. For example, you can save always the meta field show_in_news for match-report custom post type, even when the value is false. Then, you can include posts if custom show_in_news doesn’t exist (standard posts) or if it exists and it is "true" for custom post type:

$queryArgs = array(
    "post_type" => array( "post", "match-report" ),
    "meta_query" => array( 
        "relation" => "OR",
        array(
            "key" => "show_in_news",
            "compare" => "NOT EXISTS",
        ),
        array(
            "key"   => "show_in_news",
            "value" => "true",
        ),
    ),
);

This should do the job.