There’s instances of Luffy getting hurt by blunt attacks before haki was a thing. He is not immune to it, just highly resistant. At the end of TB he was literally unconscious, how do you think that happened if he is immune to all sorts of blunt damage ? The island splitter isn’t the only attack you can scale on lmfao,Katakuri literally hits harder than G3, Chinjao and Sai.
Btw, glass cannon implies both low durability and endurance. I brought up Genos because all it takes is a few hits to break down his cyborg body generally. Luffy takes damage from weak attacks, but his pain tolerance,endurance is extremely high, just like Whitebeard, these ain’t glass cannons.