Well, Limbo is "stored" on that dream sharing device that they use. I don't remember what it's called. They say so when they are attacked by DCs in the rainy city and Saito is dying. Cobb isn't using that device when he tests the sedative. No Limbo is stored in whatever device he uses then. So Limbo can just as well be a dream version of Mombasa
I want to be proven wrong, though, so there is hope that the ending isn't a dream ^^
[Edit]
"Limbo" isn't a beach, it's whatever you want it to be. Bot on the other hand, "unconstructed dreamspace" would least likely be Mombasa... But he can't spin the spinning top. What other explanation can there possibly be? A hole in the story? Either that he didn't go to Limbo or that he did, and somehow Limbo was whatever he thought reality was.
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