Ooops, somebody forgot to tell that to Boeing. See for example this press release from earlier this month:

First Boeing 747-400ER Comes Together in Everett Factory

Relax, when one is talking airliner designators, there's really no consistent, long-term logic. What might have been true at one point (such as 'ER only applies to twins') will cease to be true as soon as the manufacturer decides otherwise. And then become true again at some future time, when the mood so strikes them. So one just has to laugh about it.