Lisps separate the notion of reading strings into data structures, from evaluating those data structures. So it would be easy to assume that read-string was safe to use, but there were actually some lesser-known features in the Clojure reader that allowed code execution.

No, it would be somewhat analogous a json parser that doesn't only return a passive data struct but also executes arbitrary code.

You're missing that the whole point here is that when read-eval is set to false then, precisely, you're making your intent very clear: you do NOT want to execute code you didn't write.

The problem is that there's a hole due to Java constructors being called (even when read-eval is set to false).