I’m likely missing something, but the addon you mentioned is just a way to obtain an Auth0 subscription (which does somehow also include Webtasks as a way to run rules, custom database scripts and hooks for example). Have in mind that going with that addon you’ll be managing the Auth0 subscription from Heroku itself which may or may not be what you want. For complete flexibility you can subscribe at Auth0 directly and still use that subscriptions in Heroku deployed applications.
In addition to that you can also get just a Webtask subscription at webtask.io which would be independent of any Auth0 subscription and can also be managed independently. If this does not clarify your question then you should make your question more clear.