Why is the text displaying, but not the textarea field itself?

This is a PHP syntax issue, you can’t put a ternary operator inside an echo. Assign the value to $meta in one line:

$meta="" == get_post_meta($post->ID, 'twitter_embed', true) ? get_option('my_option_name') : get_post_meta($post->ID, 'twitter_embed', true);

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