Actually that was pretty well justified by Brooks. A tank shell explodes and can disembowel you, burn you, suck the air out of your lungs, burst your veins with the pressure and break your legs with the damage, to say nothing of knocking you unconscious or putting you into severe shock - but none of that will stop a zombie. Nothing short of severing the spine will immobilize one, and nothing short of cranial trauma will kill one, and short of a direct hit neither of those are guaranteed from an exploding shell.
The blind gardener was bull****, I'll grant you that, but I sort of took that with a grain of salt. Max Brooks wrote an entire world full of real characters suffering through the Apocalypse and kept them all beautifully grounded in the real world. And then he said, "Now, I want a blind samurai to kill some zombies." Can you really find it in your heart to deny him that? I figure he's allowed one freebie. And let's be honest, it was a fun freebie.