CB Activity doesn't output a notice when rendering comments for articles. This might be available when public/anonymous commenting functionality is implemented. So for now you'd need something else that acts on Joomla article events to add the notice. You can probably do this using CB Auto Actions with the following.
Global
Triggers: joomla_onContentAfterDisplay
Type: Code
User: Self
Access: All Non-Registered Users
Conditions
Field: Custom > Value
Custom Value: [var1]
Operator: Equal To
Value: com_content.article
Action
Code:
Code:
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Output
Display: return