Idempotence does not mean immutability, it means that two or more identical operations have the same effect on the resource as a single one. Since GET operation

Idempotence does not mean immutability, it means that two or more identical operations have the same effect on the resource as a single one. Since GET operations, by virtue of also being safe, generally have no effect at all, this is almost always trivially true. Just because the resource's content changed for some other reason doesn't mean GET is not idempotent.