"The tribesman migrated from a different land where they subsided on roots, fruits and lizards"
All animals killed for food by humans are bled. Chickens, guinea pigs, snakes and even lizards are bled. The Aboriginal Martu people of Western Australia hunt goanna (monitor lizards) with fire and spears, but they still slit their throats to bleed them once killed.
The only way they wouldn't know to bleed them, is if everyone else in their world were obligate vegetarians, and they had only started hunting Giba very recently.
Even if they had been hunting Giba for five years, they would've noticed that the ones who died from massive blood loss would taste better than those that died from blunt force trauma.