TypeScript can't always save you against yourself. But adding in the TypeGuard makes the intent explicit. If your guard isn't actually verifying the thing has the structure you say, of course it's going to fail. But at that point your code was going to fail anyway (say if you just cast it to the type), with a less-useful error message.

I don't quite understand the criticism. You may as well say that unit tests are just as unsafe as not writing unit tests, because your test might not do what you think it's doing.