The fact is that TCP was the MVP for reliable streams of data on the 1970s internet, and is woefully inadequate for modern usage, but we're stuck with it (including its oft-disabled parts like Nagle's Algorithm) because it's what's available everywhere.
Game network protocols are pretty easy in comparison with some of the protocols I've encountered in my day job, especially once you've learned the lessons of people like the author of that article.
The meaning of "custom" in this context was to distinguish it from a full TCP or raw UDP. Not to imply that there's no standard definitions for VOIP protocols. Quite the contrary in fact. The point being made is that UDP is just a substrate to create other protocols that also need to be designed for a purpose. And just calling all these protocols "UDP" is a mistake.
Btw, not to be argue from authority, but to back my words with some context, I'm responsible for at least one such "custom" protocol myself. https://packetzoom.com/blog/