That is correct. It also means that your nimbus user will then be given access to all of the data available in your UIM database which would normally be restricted when logged in as the account contact user.
In the former flash Dashboard Designer portlet, it was possible for account/contacts to design dashboards of their own. (this is the reason why you may see "dashboard designer" ACL)
The old flash dashboard was composed of dashboard designer + custom dashboard portlets. One for designing, one for visualizing the dashboards. The HTML dashboard is one single portlet that has both a design/working mode and a live/presentation mode.
With an account/contact, no matter what ACL you assign to it, it will only be allowed to see the Live mode of a dashboard that has been published to its account.
This restriction is in place because while using the Dashboard portlet the user is exposed to the entire UIM database. The new Dashboard portlet was designed this way in order to prevent account contact users from accessing other account contacts' data thereby preserving data segregation.