They're on the right track but picked the most superficial explanation. Those scenes don't exist to definitively prove reality is another level of dreaming but to get us questioning what is and isn't real the same way Cobb does. Remember his opening speech: an idea is like a virus. The idea Cobb planted in Mal's mind has now come back to infect his own subconscious.
One of the criticisms I've seen leveled at Inception is the dreams aren't dream-like enough. But that's surely done on purpose. A dreamer might not notice the reality-bending non sequitur landscape of their dreams but the audience surely would. Instead of thinking Cobb is dumb for always spinning the top we're in the same boat. Is it going to keep going this time or topple?