1. OP article doesn’t seem to mention bitcoin, and original hack covered multiple cryptocurrencies (including bitcoin).

2. Other theoretical ways would be if a governing body for one of the other cryptocurrencies reversed a transaction, such as occurred during the earlier DAO hack. Another possibility is if some of the funds were left in an exchange’s custody - obviously a hacker would work to avoid that, but maybe they were rate limited or something before an exchange locked the funds.

Some of the coins like Tether have been speed running a less trustworthy version of central banks and governmental monetary policy by allowing certain privileged users to freeze funds.

It’s probably those that have been seized, as the heist wasn’t entirely BTC.