It's not transit. Transit is about taking your data across a network to some other network or networks. These are access fees aka termination fees demanded to deliver data to a networks own customers on net.

Net neutrality bans access fees as a kind of blocking and throttling.

Twitch's servers almost certainly directly connected with the three largest ISPs in Korea. And Korea has a regulation that requires them to pay those networks and it looks like the fees that were demanded were way too high. And that's what you get in an economic system where monopolists (ISPs have a terminating monopoly over there) are allowed to set prices.

Because transit is expensive due to peering fees that Korean ISPs charge? Net Neutrality rules in 2015 did govern peering (but not nearly as much as other aspects of the internet).