Quick read: https://tutorial.ponylang.org/types/actors.html
I think one could implement a similar system in Nim using the primitives that you posted, plus some kind of channel, but:
1. Such a system is not included in the standard library (?)
2. Even if a package containing such a system outside the stdlib were available, it would not receive the same application and usage by the Nim community, as Pony actors will in the Pony community
3. So, Pony is better suited to foster a community for highly concurrent programming