I don't see a 1st amendment issue here personally. Not remotely.

If I do not agree with how a store is doing business ethically, may I walk up to the front door, padlock it, and close it down for as long as I like or until the owner manages to find a pair of steel-cutters to break it off?

Do I have the right to padlock the doors of companies that merely do business with said store?

Of course not, yet this is precisely what this DOS was doing, except lets not forget that even the weapon itself is comprised of computing power often stolen from other people without their consent.

Whatever point was being made could have been expressed with valid 1st amendment protection. Would it have been as effective? Depends on many things like the strength of the speaker's network, their communication skill, how effectively they crafted their message and supporting arguments, how well they were able to get their audience to engage, etc. But the bottom line is that they knew that by using the methods they did they'd get the easy publicity and exposure they wanted.

Obviously illegal, obviously not constitutionally protected, and obviously effective, so they were fine with it. That was the calculation as I see it. Nothing wrong with that, some of the best civil disobedience leverages breaking the law in order to amplify your point, but please, -let's not pretend that this is protected speech.