... generally speaking the best way to defend against this is to not be known to have a large amount of coins. ...
This.
The victim in this case was a bit reckless, he change the name of his company to "Aston Digital Currencies Ltd" in Sep 2017, the registered office address is his home address. Not very smart, but in his defence, it's the first case of such robbery in the UK so not something that was easy to predict.
All the self defence suggestion (gas pistol, knife, pepper spray) are bad idea when facing 4 armed (dark-skinned) attackers and when you have small child in the house.
But what I'm more interested in is what security feature could be implemented by exchanges to prevent gunpoint-forced transfers. Opt-in 3 days delay for withdrawals above certain amount? For business-owned accounts - requirement of two 2FA authentications?