They have $1.7b in the insurance fund and take in $300-500m / yr (easy calculation if you know their fees and volume, which is known to be "real"; Bitmex is absolutely legit and it is disturbing that charges against their founders continue)
When you say "absolutely legit" what exactly do you mean? That they clearly have anti-money laundering programs in place?
Or do you mean they are "absolutely legit" in the sense that they created a foreign entity that shouldn't be held to U.S. anti-money laundering laws?
They are absolutely guilty of what they are accused of here - they allowed US persons to trade in violation of US futures laws. They were trying to play the "we block US IPs, therefore we block US persons" card, which is laughable enough that they were never going to fool anyone serious.