You're right, it's TTL not LRU. The authoritative server publishes a TTL with the record — "you may keep this for N seconds" — and when it's up the resolver has to ask again no matter how popular it is.
LRU does show up in resolvers, but as memory management: cache is full, evict the least recently used early. It can only make something disappear sooner than its TTL, never later.
It's time-based because there's no back-channel. An authoritative server has no idea who's holding a copy of its records, so it can't push an invalidation.