From my understanding there is different graded of acl injuries.
Its important to know the acl is made of 2 portions.
1. Sprain: with time it heals and provides normal stability in the knee
2. Partial tear: a tear to the fibers of 1 portion of the acl. Depending on the type of tear will depend whether the knee can be functional.
Most athletes with partial tears still require surgery HOWEVER some are lucky enough that it heals and provides normal stability.
3. Complete tear: requires surgery
Looks like Tangainoa is a lucky man