Technically, chocolate will kill Adora, too, if the dosage is high enough. That box of chocolate will most likely kill neither (assuming Catra has the same chocolate tolerance as a cat), though Catra might not feel so hot after eating it.

We’re not entirely sure that is accurate as technically Catra isn’t fully cat or at least will not have the same problems as earth cats since they tend to be much smaller and have limited toerances for chocolate. Whereas we assume Catra would have somewhat different biology, else she would have to have taurine else she would go blind, meaning her trek through the desert would have been a lot harder.