There's a fairly long grace period after initial expiration when a domain is expired but can't be re-registered. It allows for the original owner to get it back (for a larger fee) before it's sniped by the next person.
Their twitter says it was on auto-renew, and their domain is through the same company as their hosting. If their autorenew didn't go through, their hosting probably wouldn't either, so it's hard to believe that their host would have let them skate by for five months without payment.
http://www.icann.org/en/resources/registrars/accreditation/e...