In all likelihood it is a bit of column A and a bit of column B. The server drops always happen at the busiest times on the server, which could be either the DDOSers picking those times in particular or the servers just buckling naturally.
But they also are not doing anything to stop it.
I used to play WOW and a few years back they had a huge problem with DDOSing, I think during Legion? The impact was very different, though. The offenders took the servers down for practically days at a time until they were properly rebooted to relieve the stress, then they started all over.
But they fixed it.
I am not someone with deeper understanding of how these things work but I have never played a game that has had such consistent problems for so long. From my understanding and how things were explained back then the best way to deal with DDOS issues is upping server capacity and having a different kind of queue, not directly connected to the main server load, so that if the login queue crashes the game does not poop its pants at the same time.

The almost on-the-hour disconnects do look like DDOS but it could also plausibly be people logging in for specific timed things. With how shoddy the servers are - let's not kid ourselves, this has been an issue since, well, always - that could also be the cause.
Looking at the player distribution on NA servers it is pretty clear that, if they are perma-locking specific worlds (all of aether and pretty much all of crystal) that means they can tell the server will not be able to handle more, which is odd given that the servers are overall emptier than they have ever been. Even in the evening everything but the main aetherytes in the three capitals is dead. You can see it in the way the marketboard flows, too, and in how empty FCs are.

Active subs sadly, in the case of this particular MMO, due to the way they handle housing, does not realistically count players. Me and a friend took a year break and we had subs because we owned homes. I have been combing over the forums since I returned - mostly because of this exact problem - and we are clearly not the only ones.

So in conclusion.
It does not matter if it is outside meddling.
SE is clearly not doing anything about it. They have made pretty clear that they care very little about their western audience despite the disproportionate amount of players here vs on the japanese servers. That kinda is an issue with non-western gaming companies in general. The fact that they do not have native representatives here who actually have decision making powers and influence the way the game is being developed and optimized speaks volumes. CC tickets going overseas as well - why? All of this leads me to believe that we are having an issue of being abandoned to our troubles, not that they are powerless. We are paying them, that is all they care about. It sucks because I adore this game and I promise this criticism is coming from a place of love.