I believe the article is written from a lisp point of view where it interactivity is inspecting and modifying the application itself while it is running. It’s assuming that the software has already been written with no existing concurrency issues, but when a person comes along and does something unexpected and rewrites pose of the application or in the example they gave, modifying a hash map, things break.

They used C as an example because under normal circumstances you compile a binary and don’t modify it at runtime.

I’m not doing to defend it to strongly though, I _think_ that is what they were getting at, but to be honest I found much of it confusing.

If the author wants to take "a C program with concurrency and mutability is fine" (correctness being left to the reader), then he might as well just put a big global lock around the REPL and declare interactivity fixed too!

It's especially weird since the author is talking about inspecting and modifying data at runtime; gdb happily attaches to a process running a program written in C. However, if he were talking about inspecting and modifying the program itself, then we'd be cooking.

I think the example makes it pretty clear that this is exactly what is meant. I was thrown by the term interactivity at first as well. But I think the trichonomy is sound, and the Erlang Vs Python Vs Rust buckets are quite meaningful, though I would argue it's rare for people to "interact" with running Python programmes in this way.