HTTP headers are different than the HTML head. The meta tag you’re looking to add (for the Twitter card) is actually a meta tag that gets place in the HTML head.
So to achieve exactly what you’re looking to do from a PHP “functions” file, you’d want to hook into wp_head instead. Like this:
add_action('wp_head', function(){
echo '<meta name="twitter:card" content="summary_large_image" />';
});
You can piggyback on a single hook to add as many meta tags as you need (since I see you’re declaring the large summary size for Twitter, you’ll probably want to tell it which image to use as well. You can do it like this:
add_action('wp_head', function(){
echo '<meta name="twitter:card" content="summary_large_image" />';
echo '<meta name="twitter:image" content="' . get_the_post_thumbnail_url() . '" />'; //The post thumbnail is the featured image– you could change this to a different image if desired
echo '<meta name="twitter:creator" content="@yourtwitterusername" />'; //You can add the Twitter username of the author here (by hardcoding or even pull from the author's profile)
});
Think of the WordPress actions (or “hooks”) as points when things happen– so the above code just tells WordPress: “hey, when you trigger the wp_head action, run this code as well”.
Here is a list of all (or most) of the WP actions. I use this list all the time, so it’s worth a bookmark.
Remember with filters you need to return, but with actions you just do what you want to happen at that point (which is why we are just echoing here).
Sidenote: I know the is_page() function accepts strings, but using the page title here can be fragile in case the title ever gets changed. Consider using the post ID there instead– so like is_page(123)
Hope that helps!