As far as I can tell, that's not what the great-grand parent comment was about.
Have you ever programmed in Erlang?
About your first paragraph: not all databases should be append only, but it would be a good idea to make append-only the default and only deviate for a good reason. (Instead of making mutable the default, and only weirdos go for append-only.)
I understand the point as: being able to send a message to a particular actor, passing it a value that you can later retrieve by sending that same actor another message, is semantically exactly the same as calling a setter on an object reference, and later calling a getter to get the value back. In both cases you have a pocket of mutable state, potentially accessible by multiple unrelated places in code, that isn't reflected in function signatures.
Mostly true, minus the fact that the mutable state's access is serialized / centralized due to the nature of Erlang's actors (lightweight threads; usually called fibers in other languages, and even that is not a good analogy since they are preemptive and not cooperative). So the semantics being similar is not strictly and 100% true. You can't do non-atomic volatile modification like you can in C/C++.
Have you ever programmed in Erlang?
About your first paragraph: not all databases should be append only, but it would be a good idea to make append-only the default and only deviate for a good reason. (Instead of making mutable the default, and only weirdos go for append-only.)