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.
The passive voice sentence would be: "Byl sežrán nějakými pavouky."
But Czech would be much more reluctant to use the passive voice here that English. If I had to translate the passive voice English, I would use the active voice in Czech in this sentence and similar ones.
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. :-)
Well, yes. The verb "sežrat" is perfective (that's its aspect), which means the action is completed. The prefix "s(e)-" here adds the meaning of "eating the whole thing", similar to "eat/gobble up".
On the other hand, the continuous tense in English here would suggest an ongoing, unfinished action, which in Czech would be expressed using an imperfective verb.
One way to get the corresponding imperfective verb is to remove the prefix, i.e. "sežrat" -> "žrát". Hence "Žrali ho nějací pavouci" means that some spiders were eating him, but haven't eaten him completely.