Ultimately, the spiders won. But even in English the two sentences are different, one being in the active voice, the other in the passive. The Czech sentence here is active, so I would suppose the translation should be active as well. But you could certainly Report your version, if you haven't already done so, and see what The DL Powers That Be think.
Yes, spiders bite, but they also eat. The verb in this sentence (sežrat) specifically refers to eating. The fact that it's a big job for some spiders to eat a whole person doesn't necessarily make the sentence wrong... and for some, goofy sentences make learning more fun. :-)