A REPL, by its name, is a very narrow version of the broader paradigm of interactive computer programming environments. But Notebooks are not REPLs, unless you use REPL to mean "interactive programming environment" and not REPL. Notebooks are much broader than a REPL! In a notebook, you can go back and edit and run individual lines of the notebook without re-running the whole notebook from the start and without re-computing everything that depends on what you just edited. Behavior like this makes it super hard to track the actual state, and super easy to lose track of how things are how they are. That's pretty terrible!
A REPL, by its name, is a very narrow version of the broader paradigm of interactive computer programming environments. But Notebooks are not REPLs, unless you use REPL to mean "interactive programming environment" and not REPL. Notebooks are much broader than a REPL! In a notebook, you can go back and edit and run individual lines of the notebook without re-running the whole notebook from the start and without re-computing everything that depends on what you just edited. Behavior like this makes it super hard to track the actual state, and super easy to lose track of how things are how they are. That's pretty terrible!
The parent article links this great talk that goes into more detail than the parent post and is much easier to understand: https://www.youtube.com/watch?v=7jiPeIFXb6U