It's not a poor answer it's a very brilliant one, gojo did not let his guard down because he let his guard down per se, he probably sensed the slash but did not dogde it that's what he means by let his guard down, it's the same reason gojo does not dogde punches, because of his infinity he saw the slash but because he destroyed mahoraga there was no way for sukuna to bypass infinity so he just stood there knowing his infinity will take care of it
