> It’s always going to be orders of magnitude more difficult/expensive to retrofit testability.

I can't take this any other way than a huge overstatement. I'm yet to find any such example where retrofitting it is orders of magnitude harder.

Worst case it requires understanding the entire system again, and adding instrumentation here or there. That can be almost as hard as writing the entire system again, but usually it's much easier, because all the exploratory work has been done and tests are merely formal.

I'd say that correcting the bugs you'll find with those tests is way more work than adding the tests, and maybe they would be easier if they were discovered earlier. But again, the "orders of magnitude more work" is an overstatement.