From your example:

> DELETE /idX/delete HTTP/1.1 is idempotent, even if the returned status code may change between requests:

So requesting to open the garage door multiple times which results in an open garage door in the end is an idempotent request, even though after the second request the response is "I am already open!"

Now, a request to toggle the state of the garage door would not be idempotent. The state of the system is different if you call it an odd or even amount of times.

In general, an idempotent method is a method such that calling it one time is the same as calling it n times in a row for any n > 1. However, idempotency doesn't require that calling the method one time is the same as calling it zero times, although such a method isn't excluded from the definition either. So a stateless method is necessarily idempotent, but an idempotent method isn't necessarily stateless.