How to add replyToUrl schema.org to WordPress comments?

You could instead try the following replacement:

/**
 * Add itemprop attribute to the comment reply link
 */

add_filter('comment_reply_link', function( $html )
{
    if( false === stripos( $html, 'itemprop="' ) )
        $html = str_ireplace( '<a ', '<a itemprop="replyToUrl" ', $html ); 
    return $html;
}, 99 );

through the comment_reply_link filter. The HTML generation, of the comment reply link, only supports a given set of attributes and itemprop is not among them. That’s why we try the replacement here instead.