Not really. You can do actor model with 1:N or 1:1 threading. With 1:N you get actor-model concurrency without native-thread overhead per actor, but no parallelism; with 1:1 you get parallelism but you have native thread overhead for each actor. With M:N, you can limit your native thread overhead to whatever is useful given available hardware, while scaling out to as many actors as you need.

OTOH, M:N and 1:N both require you to do more to handle actors in the runtime, which potentially adds overhead to everything, M:N may not always be a win compared to 1:1.