Dolphins are clearly intelligent, but not smart enough to organize a colony of many thousands of permanent shelters, and share community surplus that advances a dolphin civilization that benefits those who participate.

So, probably as intelligent as nomadic pre-historic humans. I wouldn't rate that as "highly" intelligent. Just somewhat intelligent.

These are still pre-historic dolphins, since they don't seem to record history for themselves yet. If they carry oral traditions, they seem to be limited to hunting techniques, and not specific events and personalities measured across time. There is no evidence of symbolism invented by dolphins.