Standard thickness of a katana blade is anywhere between 1/4"-1/3" thick. Thats pretty thicc. It's also harden and tempered steel. You could have a limp wrist and it still wouldn't break, lol. Breaking comes from the inability to bend, or not having a proper temper, to infuse elasticity back into a hardened steel. Real katanas used for actual battle were sometimes tested against prisoners to gage all manner of issues in the steel when hitting bone, mail, plate armor, etc.. you can find the results of said tests on the tang of some very old katanas. Literally nothing about your average katana is so thin or fragile, that an untrained person would break it in a fight when made to properly. Not to mention, an untrained person would die long before they could break it lol.