Recall also that Smalltalk had a context, or a world, that included the IDE, and it was primarily driven by interacting with visual components in that world to which you would add methods (more like event handlers) while the whole thing was already running. I think this is a missing piece in properly understanding the context of his comments since most of us have only ever experienced coding in a text editor.
I don’t think Kay had the Actor model in mind, at least I’ve never heard him acknowledge that as a direct influence, though there exist a number of similarities. He instead claims inspiration from biology. Erlang started as an implementation of Smalltalk ideas and adopted the Actor model as they moved forward. Is that what you are thinking of, perhaps?