Boy has an iron chin no doubt, Porter, Wach, Garcia, Golovkin and Canelo have the best chins in the game.
I think it's essential you have a good chin when you fight with that style. He'd probably be a CW if he was in optimum shape and he has quite short arms and falls off balance sometimes and leaves himself wide open after throwing his right hand. But Martin is a big strong dude and he caught Kownacki with some heavy shots which he took very well. Polish HW chin rankings Wach Kownacki (as of now) Szplika
And he throws 77 punches a round. This guy is going to make some noise, fat belly and technical deficiencies not withstanding.
His chin definitely isn't glass. Martin might not be as big a puncher as his record suggests as he's not beaten or stopped anyone of note, but he's at the very least a solid puncher. Martin himself showed a far better chin that the Joshua fight showed. Either Joshua hit's that hard or perhaps Kownacki isn't that big a puncher. I suspect the latter, remember Szpilka isn't known for being durable in the slightest. Kownacki like Martin is probably a solid puncher, not a huge one punch hitter. But with his work rate if he lands enough he'll stop almost anyone. Would love to see Chisora vs Kownacki that would be an action packed fight.
Chisora vs Kownacki would be very winnable fight for Chisora who just had KTFO'd far superior technician Carlos Takam.
What about the prototypical Polish warrior, Andrew "King Kong Killer" Golota? Still the greatest fighter this country ever produced? He'd run right through the current hw division with his chin sticking out and his guard down.