When Erlang was designed/invented the goal was never to make a new language, the goal was to design a way of building systems with a set of specific characteristics, massive concurrency, fault tolerance, scalability, etc. One part of this was the language, Erlang. But at the same time we were developing the language we were also looking at how you would use the language and its features to build such systems. The language and the system architecture went hand-in-hand with support in erlang for the type of patterns you would need in the system architecture.
These ideas and patterns existed before OTP. So there were Ericsson products built on top of Erlang which used these ideas and patterns before OTP. OTP "just" took these ideas and design patterns and formalised them in a generic, cohesive way. Plus of course a lot of useful libraries.
So we were never really out to design a functional language based on the actor model, we were trying to solve the problem.
(We had actually never heard of the actor model but were told later that Erlang implements it)