The biggest hack by far was Coincheck. I'm unaware of any other Japanese exchange getting hacked post Gox. It's a largely Korean tradition.
Coincheck were not regulated and all of their stolen XEM was in a hot wallet. None of it was cold. None of it was multisig despite XEM having it baked in to the protocol. One Trezor would've prevented everything.
Because of that even if they were insured no one would ever have paid out. Most of it was their own money and so there wasn't a huge proportion to pay back to actual customers.
I dunno about Japan, but the insured places in the US only cover USD balances like any other bank. The crypto insured places like Coinbase only cover the 2% in hot wallets for hacks at their end. The 98% cold stuff is not insured.
Interested to see what the deal is with this policy. I wouldn't want to put it to the test. An insurer's first job is to find a way not to pay.