It can be offline. to quote the rules:
"All games must include a leaderboard system (offline or online) so that the players can compete for a high score if they happen to win the contest. We suggest you check out LootLocker for your leaderboards as they can be added to your game really easily."
HOWEVER. If you score in the top three games, you're required to add their online scoreboard."The top 3 winning games are required to implement Lootlocker leaderboards after the rating period to ensure score accuracy."