Interesting. The article states "The compiler prints the value as a given variable changes." -- surely it means the program does, and not the compiler?

I take it to mean that the compiler inserts variable print code on variable modifications.

It might be more precisely stated something like "The language's semantics require that when a variable changes value, that change includes the side-effect of printing the new value."

The compiler produces p-code to be interpreted by an interpreter, so it is the interpreter that prints the value.