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Coinbase mentions a few reorgs without double spends, and a quite a few more with: https://blog.coinbase.com/ethereum-classic-etc-is-currently-...

IIRC selfish mining is mining without broadcasting the blocks. Some pools do it for only a short time to get a headstart on mining the next block, the obvious risk being someone else broadcasts a block before you do and your block becomes orphaned. In the case of a 51% attack, I believe you can simply selfish mine then reorg the chain every now and then because your chain is longer. This way, you mine every single block.

So this is a publicity stunt intended to show off their mining rigs.

Eyal, Ittay, and Emin Gün Sirer. "Majority is not enough: Bitcoin mining is vulnerable." Communications of the ACM 61.7 (2018): 95-102.

https://www.cs.cornell.edu/~ie53/publications/btcProcFC.pdf