You have to consider that there is actually no true message passing in Smalltalk. Smalltalk just uses that term for dynamic dispatch of compiled methods and calling them (synchronous local invocation). Since Smalltalk 76 methods are compiled and there is a dynamic dispatch mechanism similar to Simula 67 (or Java); the main difference is that Smalltalk uses an internalized pointer to the selector (i.e. method signature) for dispatch instead of the more efficient method index used by Simula 67 and most modern OO-languages (including Java); essentially all OO languages support late binding. When you read e.g. https://dl.acm.org/doi/10.1145/3386335 (highly recommended), Ingalls describes that the first Smalltalk implementation in 1972 was a bit closer to "message passing" in that token streams were sent to an object for interpretation, but that was synchronous and local as well (like the later method dispatch) and the objects were not "active" (in contrast to e.g. the Actor Model as implemented e.g. in Erlang). I spent a lot of time building tools to study the internals of Smalltalk-80; if you are interested in the technicalities, here is the link: https://github.com/rochus-keller/Smalltalk/.