That's quite possible, but this group had previously announced they'd be doing this so it's a reasonable possibility.
Also, note Nintendo (Mario Kart and Smash) never went down during those games launch and is still up. The player count isn't as high, but they could play.
Well, either that, or the group announced that they'd attack those services today because they figured there was a good probability that everyone trying to log on with their new Christmas goodies would cause service problems they could take credit for.
When the PS4 first came out, it was a royal mess to get online just to get connected and activate the console. Seems they were also targeting Bungie's servers, but their server / network system has been a complete mess since they released the Destiny console game.
