Wanted to get rid of Paragrapgh tag in submit buttom of the WordPress form

If you control the theme the simplest way to fix this is passing the submit_field argument into comment_form():

        'submit_field' => '%1$s %2$s',

Default: <p class="form-submit">%1$s %2$s</p>, where %1$s is the submit button markup and %2$s is the comment hidden fields.

which removes the <p> altogether. However it might be useful to wrap the button and hidden fields in a div instead, e.g. 'submit_field' => '<div class="form-submit">%1$s %2$s</div>'.

As above the problem with your code is you’ve got the filter input and output arguments wrong: comment_form_submit_field has inputs $submit_field (string), $args (array), and should return a string.