I was curious about the cost as well, so I ran some rough numbers.
417.5 BTC/GH/day for equihash if you're renting from nicehash [1]. Block interval 10 minutes [2]. 144 blocks per day target yields 0.0869 BTC/block, so cost of the 22 block reversal was ~1.91 BTC, or roughly $15k.
I'm curious if this was done as one large deposit, or many smaller deposits. I've imagined a system where block confirmations required are based on a computed cost of attack done like I did above, which would be pretty effective for very large single transaction double spends. A bit trickier to handle multiple deposits spread across multiple user accounts.
[1]: https://api.nicehash.com/api?method=simplemultialgo.info