Yes, ML is adding new constructors to exn type at runtime, but this is different from GHC implementation of dynamic typing which uses typeclasses, because GHC does not add typeclasses at runtime. Take a look at ezyang(who is a GHC developer)'s comments in the post.

I repeat that this is extremely obscure and probably only interesting to people like ezyang. He is engaged in the discussion, so I guess the post was not pointless.