Their attacks SEEM (I know this probably isn’t the case) to be unblockable.
Is this how you have to play to have a high level of skill at this game? If I don’t want to look like a retard, can I ever compete against these guys? What exactly is going on here and how do you counter it?
Again, kudos for asking about this playstyle and how to counter it. There has been some good advice so far which is mainly to work on fundamentals like timing parries, managing stamina, practicing dragging, keeping correct range, etc. Focusing on these will grant you the most gains.
As for your other questions: the attacks you are seeing aren’t unblockable, and you can parry the attacks all day without looking like a retard. But when it comes to attacking them back, you’ll find that people have pushed game mechanics to the extremes and you may need to use some wonky techniques or else you’ll never get a hit on the good players.
Some of the things you may be seeing are reverse overheads where they turn 180 degrees look up and do an overhead attack, which is a very fast attack. Another could be a stab drag where they’ll stab to your side and drag it in around your parry. When crouching and looking at the ground, that will be the fastest non-reverse overhead you can do, so that’s why you’re seeing that.
Even past this are the minor combat enhancements like animation hiding. A couple examples of this would acceleration of a polearm riposte. Since you are turning with the animation, it is hard to tell whether an attack is happening or they are merely turning toward their left. Another example would starting a two-handed sword stab while turned slightly to your right. In this way, your body hides some of your sword making it harder for your opponent to see your actions.
Practice, practice, practice. Also the 120+ FoV suggestion lets you see more of the opponent’s body, giving you more information and better chances of correctly responding to them.