All this atom does is to make the normal Joomla "system-messages" area appear in a popup - if your third-party extension does not use that then the atom will have no effect.
I have just added this on another site and I was testing it with the Contact Form. For all the fields, including any custom fields, it would show the modal if the fields were invalid.
However, after I successfully sent a message using the contact form, the Message "Thank you for your email." was displayed at the top of the screen normally instead of appearing in a modal.
Since this is a Joomla "system-messages" is there a way to get this to also display the default Joomla "thank you message" inside a modal window?