But If he was positioned on Bitmex to profit from the mass long liquidations, it’s a different story. This may have been a way to sell thousands of coins to fiat while simultaneously recouping them with shorts on Bitmex.
The wildest speculation so far that I have seen is that BitMEX was behind the dump to liquidate the crap out of the long positions, but another conspiracy theory without proof to back it up.
Overall, having that many coins by default grants you a massive advantage over the rest of the market due to how thin the books are, so from there it makes sense to have a leveraged side trade on BitMEX.
There are no set rules that makes side trading illegal or whatever, so traders are free to do it. People here are upset that the price went down instead of up, that's why they call it manipulation.