So the DDoS attacker was targeting the Fediverse!
What's the clue? They targeted joinmastodon.org.
That's pertinent because joinmastodon.org is the site where new users go to find a suitable instance.
In other words, this wasn't just about someone with an axe to grind against @Gargron.
They were actively trying to prevent people from registering for a Mastodon account on *other* instances.