I guess the key is whether this event is arranged by meta.discourse.org or by CDCK itself (if those entities are truly separate).
Personally, if I RSVP’d within meta.discourse.org I’d expect my data to be utilised for something like this. Ditto for a zoom.org webinar ‘owned’ by CDCK.
In my domain (health) ‘privacy’ is often used as an inappropriate excuse by those who don’t want to go to the trouble to safely share data in ways that benefit the user (in our case the patient). And patients generally suffer as a result while the corporate butt is covered. Not cool.