Is the extension purely for maintenance?

No, the beta was supposed to end today. Extension will be at 5 PM Pacific due to the inability to connect to servers early on.

according to the front page:

February 9th Extended Schedule

Attention: Servers will shut down Sunday, 12:00 AM (PST) 12:00am (midnight) – 5:00pm Server Maintenance (Users won't be able to log in)

5:00pm Servers Open (Start of Extended Period)

8:00 – 8:30pm Giant Automata: The Awakening

10:00 – 10:30pm Elder of the Unfathomable Abyss

12:00am, February 10th Server Shutdown (End of the Closed Beta Test)