I don't show a fiddle because it doesn't even seem to work in the tinymce documentation. Please watch the demo here, in JS it says invalid_elements: 'strong, b, em, i', however you can clearly make the text bold.

Is there something I'm missing? Why is it not working?

EDIT: you can test this minimal playable code also by pasting it here

<script type="text/javascript">
tinymce.init({
  selector: 'textarea',
  invalid_elements: 'strong,b,em,i',
  valid_elements: 'p',
});
</script>

<form method="post" action="dump.php">
    <textarea name="content"></textarea>
</form>