I have no idea how to verify the statements, but I found some comments on the blockchain.info page for the bitcoin address regarding the DoS. It is supposedly from the blackmailers: https://blockchain.info/address/1FxHcZzW3z9NRSUnQ9Pcp58ddYaS...

"Somebody with great power, who wants ProtonMail dead, jumped in after our initial attack!" "We have no such power to crash data center and no reason to attack ProtonMail any more!" "WE DO NOT HAVE THAT POWER! NOT EVEN CLOSE!" "We are not attacking ProtonMail! Our attack was small, directed at their IP only and lasted 15 minutes only!"

I don't believe Protonmail have said they have received any more requests for money, so that would go along with the above. I agree that it was silly to pay the blackmailers, but there is some reason to believe that these are two separate attacks.