How do you know that the data have actually been deleted?

The excerpt from the (apparently edited) Meetup response merely says, "Once a Meetup Group has been removed from the site, it's final."

That doesn't say anything at all about the state of the data. It may just be that they don't want to have to deal with appeals when groups are removed, so their policy is to make all removals final, even if the data are left untouched.

That makes a lot of sense, I hadn't considered that it might be a decision based on the high support costs of ever entering into negotiations rather than a technical limitation or process issue.

That's a good point; on Meetup's scale, they probably deal with a large amount of similar disputes. It's still a shame that they don't make exceptions for cases like this.