This is intended behavior: the purpose of the type check is to see if the given object is assignable to the given type.
In your case, the object breaks is assignable to Example, i.e. it fulfills the contract that Example defines as an interface.

Typically you would write this manually yourself in the following way:

function isExample(obj: unknown): obj is Example {
  return typeof obj === 'object' && obj !== null && obj.exists === true;
}

const breaks = { exists: true, doesNotExist: true };
isExample(breaks); // true

In other words: the type check only checks for soundness, but not for completeness. Or: breaks has to be assignable to Example but not Example has to be assignable to breaks.

That being said, it could be possible to add an option to the transformer that makes it check for additional properties and disallows them, something like allowAdditionalProperties: false.
However, I haven't investigated how difficult this would be/if possible/how time consuming.

If you need such a feature let me know, and I'll put it on the list of planned features.

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