idk if other people mentioned it for you, but kms are less than miles so 5km = 3.10 miles in radius, which is still massive regardless. Considering we saw what happened when Sukuna was killing people in 200 meter radius haha.
edit: the move shoto used is the same one he used in their last fight, yeah. The todoroki storyline is probably Horikoshi's best in MHA, no comparison honestly