8K is not required, they did a press release and its in the public domain. The SEC requires companies to file an 8-K to announce significant events relevant to shareholders. This is standard business event.
"What triggers an 8-K filing? item is triggered when the company enters into an agreement enforceable against the company, whether or not subject to conditions, under which the equity securities are to be sold. If there is no such agreement, the company should file the Form 8-K within four business days after the closing of the transaction. IMO