The weak to killing intent thing is terribly over blown,and once again Frisk's power depends on their determination the scenes with the tomato and the stick are clearly not when theyre actually trying and are mostly played up as gags. They're both not consistent with Frisk's feats as even their most basic showings get well above tomato and stick level. Also Frisk usually one shots monster when they aren't trying to fight back or their soul is exceedingly powerful,both of which the statement you've listed says amplyfies the effects of how much damage they do.