Using AEM 6.3 , I am trying to add a custom action type in AEM Form Core Component. It had only 4 dropdown items. I successfully added the custom action in the container dropdown.
Also - just so you know - the article that you referenced was for AEM Form that you create by using AEM Foundation components. If the form core component does not meet your needs - look at using the foundation form components.
I have similar requirement to have a custom action type to submit to servlet. I haven't got any possible solution. Did you get any solution? @Shalini_Gupta