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I doubt the trading volume on MXC. Their Hive wallet is not very active. Only a few deposits/withdrawals a day.

Link: https://hiveblockexplorer.com/@mxchive

I get your point and I see that too, but I would like to see a screenshot of somebody who has an account at mxc.
A screenshot of their orderbook...

Orderbooks are here:
https://www.mxc.com/trade/easy#HIVE_USDT
https://www.mxc.com/trade/easy#HIVE_BTC

There's much more HIVE on the sell side than there is HIVE in their wallet. It seems there is HIVE being sold that is actually not on the exchange.

Maybe there have more accounts and this one is just their hot wallet?
They have 11 million Hive according to their order book...
And that is only the amount of HIVE in sell orders, so they have more...

Could be, but @mxchive is used for both deposits and withdrawals. Besides, I can't find any transactions between @mxchive and another possible MXC wallet. I also searched for other accounts with 'mxc' in the accountname, but could not found any other wallet.

Finally, I looked at a list of the largest transfers per day I don't see any large HIVE transfers to a possible other MXC wallet.

I think this is their only wallet.

Could it be that they had STEEM and got the airdrop?

@mxchive was created on april 16. After the hard fork, so there was no airdrop on this account.