Hm, just seems like really bad copy. They seem to have meant that any new presentations will be public.

Prezi's response in linked thread:

This copy does not reflect our policy - thank you for bringing this to our attention, and we’re currently fixing the mistake. Any private presentations created during your trial or paid subscription will remain private unless you change the privacy settings. (1/2)

Prezi offers free licenses, and when you cancel your trial or subscription, you’ll have the option to downgrade to a free license. After that, any new presentations created from the free license will be public. (2/2)