Yes, he is and it isnt just a Tremor attack because tremor cant harm a logia. You are failing at basics.
Try to bring Meme, Kaido or King down with such attacks and you will see a difference.
I mean you already got to see just what kind of overwhelming power it takes to bring King down.
If WB's mere nameless slash did that to Blackbeard, can you imagine what Mihawk or Zoro's named attacks would do?