Entire

Yes, it's Cloudflare Durable Objects with SQLite storage. Here's how it works:

src/leaderboardDO.ts defines LeaderboardDO extends DurableObject<Env>:

On construction, it runs ctx.storage.sql.exec(...) inside blockConcurrencyWhile to create an entries table (nickname, score, total, date) if it doesn't exist — this is the new SQLite-backed Durable Object storage (not the older KV-style DO storage).
submit() inserts a row; top(limit) runs a SELECT ... ORDER BY score DESC, date ASC LIMIT ?.

wrangler.jsonc wires it up:

new_sqlite_classes is what makes this a SQLite-backed DO rather than the legacy key-value DO storage.

src/index.ts always resolves the same single instance via a fixed name:

So there's exactly one LeaderboardDO instance ("global") for the whole app — every submit and every read hits that one object, which serializes access and gives you strongly consistent read-after-write behavior for free (a nice property of the single-writer DO model), backed durably by its embedded SQLite database.