It's a different, weaker level of type safety that's become so prevalent that you probably take it for granted. The runtime checking is just to make sure that operations are performed on logical types.

My favorite analogy for this is the restaurant one. Haskell insists on having two waiters: one to deliver the food and a second to deliver the check. Clojure allows one waiter to deliver both, but throws a fit and leaves the restaurant if the check arrives before the bill. Assembler eats the first thing that arrives and signs the second, even if the bill comes before the food.

Clojure is "type-safe" in that it will exit instead of performing an illogical action (e.g. dividing a string pointer by a network socket). It's just as type-safe as Haskell code littered with fromJust.