Taking a big leap, but in order to make it more comprehensible by known association, HubSpot can be seen as a conceptual functional subset of what Salesforce is. That is to say, some companies use it (HubSpot) as their (minor) full-fledged CRM, and therefore, for any given lead or prospect qualification campaign, they may ask for specific information that is tailored to the campaign’s needs of information.
For example, one can easily envision how a given Swan Lead generation campaign asking their leads to provide their annual income, or another asking for the investment range. This data may be only demanded in certain campaigns, thus not found on all customer records (i.e. the small percentages they mention in their status release). This data will likely remain attached to the historical record of the person, as he moves from lead to prospect and then to client.
This sort of information can either be part of the predefined data fields defined in the CRM (see the default contact details
here), or managed and stored through added custom fields (see
here). This is all part of the contact data record, which APIs can give access to with more or less effort and understanding.
I haven’t seen the complete list of names of the 30 or so companies affected by the leak. I wouldn’t expect Hubspot to release it to the public, but rather it should be each affected company that contacts its own user base. There are normally regulations that delimit the timeframe to divulge this information to those affected users, as well as ethical and early alert considerations.
Judging by the time that has gone by, albeit it not being tremendous, it should have been paramount for companies to have contacted their own set of customers on the matter at hand by now. It should, therefore, probably be known by now to the general public, derived from public reports made from notified customers. The fact that the complete list of 30 or so companies it yet not known, suggests that some are taking way too long to do their part ...