"Actors" was actually one of the names we used internally for a long time (it's still all over the code), but eventually decided against because we found that people familiar with the Actor Model actually expected something a bit different, so it confused them.

But yes, the basic idea is not entirely new. For me, Durable Objects derive from my previous work on Sandstorm.io, which in turn really derives from past work in Capability-based Security (many implementations of which are Actor-oriented). But while the idea is not entirely new, the approach is not very common in web infrastructure today.

(I'm not familiar with Orleans.)