It's weird, I always kind of wondered why Erlang gets left out of the FP discussions. It's not as dogmatic as Haskell or something, but functional-ness is kind of its bread and butter.
Someone down the line probably mentioned to them "Hey, btw, did you know you have a functional language?" And they probably shrugged and said "Oh, cool!"
I would guess Joe, Robert or Mike never went about reading papers on type theory, thinking about meta-circular evaluators, or y combinators etc and said "We need that, let's implement it!". Instead, they started with stuff like "let's minimize state" so made state explicitly passed in. "Let's minimize mutable state", so make data immutable. So then need recursion instead of for loops, so let's think about tail call elimination. They played with Prolog so language inherited the base Prolog syntax which has a functional taste to it, that helped as well.
The same thing kind of happened with the actor paradigm. They never started with a goal of implemented an actor / message passing programming paradigm. It just fell out of fault isolation property. Someone later told them "Oh btw, did you know you have implemented the actor paradigm?" and they probably shrugged and said "Oh, cool!" and went back to doing whatever they were doing.
That was actually how he described tail call elimination in his LFE talk: "of course you have TCE/O, why the hell wouldn't you?"
Which was a refreshing perspective to me, considering how many programmers still seem to have been heavily indoctrinated to the idea that recursion is automatically bad.
Erland on the other hand, is targeted on soft-real-time distributed systems. How was the quote? "Every soft-realtime highly availabe distributed system contains informally specified, incomplete and bug ridden implementation of erlang." Or something like that.