He is not describing Actor Programming model. He describe his own model.
Message passing in Smalltalk predates Hewitts Actor Model and was used as inspiration. The messaging in Smalltalk is little different and IMHO better in many cases.
But he's not describing Smalltalk. He is describing what he wanted Smalltalk to evolve into. Which he didn't know how to do, because otherwise we would have it by now.
This quip by him from OOPSLA '97 is well-known:
I made up the term object oriented. And I can tell you I did not have C++ in mind..
A little less well-known are the words that immediately follow:
So, the important thing here is: I have many of the same feelings about Smalltalk
Why would you a hypothetical quote of Kay for cutting off the full context that he also has criticisms of Smalltalk, and then cut yourself before he specifies that what he's not committed to is the syntax and library system, while the message-passing execution model is the important thing he's trying to promote? That just muddies the waters more. This email was sent a year after OOPSLA 97, so clearly he can't have been talking about messaging as Smalltalk's problem.
As for where he wants Smalltalk to go, that's what Squeak was for. He talked about it on plenty of occasions, at least one of which was also before OOPSLA, and actually did get a research team together to develop it out in the late 2000s: https://tinlizzie.org/IA/index.php/Papers_from_Viewpoints_Re...
The original Smalltalk in 1972, the language of which Kay designed, indeed had some kind of message passing (even though it was synchronous, the receiving object interpreted messages composed of tokens). Smalltalk-76, essentially designed by Ingalls, who was also the sole author of the 1978 publication, made a fundamental shift towards compiled virtual methods, essentially as it was done in Simula 67 and adopted by C++ (though much less efficient). So yes, it makes pretty much sense when Kay claims that he didn't have C++ nor Smalltalk in mind when talking about OO. See also https://dl.acm.org/doi/abs/10.1145/3386335.
Message passing in Smalltalk predates Hewitts Actor Model and was used as inspiration. The messaging in Smalltalk is little different and IMHO better in many cases.