We've reached out to the developers at Niantic for its take on the claims and will let you know if it can shed light on the situation. However, it's hard to know how much of this was due to the self-proclaimed attackers. As many players can tell you, Pokémon Go has had periodic issues since launch. Although it's unlikely that numerous servers would fail on their own at the same time, there's no firm evidence of a denial of service attack beyond the alleged perpetrators' say-so.

Update 7/17: Canadians are getting their turn at Pokémon Go today.