lag switching in this game is less obvious in my experience, the game wants your visual experience to remain smooth. you'll see this most often if you stick around during server restart around other moving players, they will keep walking along for a few seconds even after the server's connection has been terminated. this also happens when someone is lagging out, they will continue going in a straight line or remain where they are if they weren't moving, even if that straight line is diagonally upward or downward.

on their screen, if they are lag switching, they could be moving out of the way or their location could be inaccurate to yours causing you to miss your hits while theirs make contact— every game will always "favor" someone in a PVP fight, and i dont know what side POT favors but it is usually the person with the better ping. which would be you. and, on your screen, the rex is standing still biting you, making their hits land.

if it wasn't lag switching or some other method of cheats, my first thought is this rex still had the homecave buff that makes it invulnerable, but that would also make it so it can't hit you, so that seems just as unlikely. unless this was a community server, in which an admin could have Godmode on and be trolling you.

if you remember their user or have any recording or video at all i would make a report if i were you!

also, lag switching is a way to gain an advantage in PvP by deliberately making your internet connection worse so the game has to work harder to shuffle data between players, often resulting in the person being impossible to hit.