I'm with the both of you. It might be an unfortunately uninformative design, that doesn't mean you have to rely 100% on design to accept a little bit of responsible attention to detail and feel the door and check for movement inside to tell you the thing is still running.
Their criticism stems more from the "don't make me think" philosophy. If you're quickly fetching something from the kitchen, it's totally understandable if your mind is elsewhere. It's not so much that it's impossible to know if it's on, it's that it could be much, much more obvious.
This is analogous to a row of unlabeled buttons that only show their meaning on hover. Sure, if the user stopped for a moment and surveyed the scene, it would be possible to know what everything does. But to what end? There are no benefits and the cost is high: you interrupt the user's train of thought. More on that:
I'm having a guilty-pleasure chuckle right now because the "kings" of UI are getting sloshed with soapy water because of a bad design decision :)