Honestly, it's not so disturbing.

He is a man who has lived through decades of fighting the Sin Eaters. He is probably the strongest human of the First.
The fact that he can fight with us as equals for a while is not so impossible (Even if we exclude Zenos, people like Gaius, Estinien, Ilberd or Raubhan for example can perfectly do). At the end of our first fight, he barely neutralized us for a few seconds with one specific technique, which will never work against us again as efficiently (proving like we have probably learned from our mistake).

If I compare Ran'jit's strength to how easily we were arrested and forced to flee during Nanamo assassination attempt, I have always find it much more shocking.