> "you don’t know the real types of variables at runtime", this is just flat wrong. One doesn't know the real type of a variable until runtime.

I read the sentence as

  "you don't know the real types of variable at [the start of] runtime" 

as opposed to

  "you don't know the real types of variable *during* runtimes"

So I understood the author to mean what you said. Could have been clearer you're right.