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Yes. I'm sure that this is all that is happening.
I just opened up my bittrex account and my poloniex account, and looked at how I would transfer between the two of them. And this is EXACTLY how to do it.
I would have to send Steem held at Bittrex to the Poloniex account on steemit, or vice-versa.
So there is no great mystery here.

Great. Good to know. Thanks