Despite my love of yaml. I actually think he has a small point with unquoted strings. I teach students and see their struggles. Bash also does unquoted strings and basically all students go years and years without realizing

cat --help
cat "--help"
# ^ same thing

cat *
cat "*"
# ^ not same thing

cat $thing
cat "$thing"
# ^ similar but not the same 

To know the difference between special and normal-but-no-quotes you have to know literally every special symbol. And, for example, its rare to realize the -- in --help, isn't special at a language level, its only special at a convention level.

Same thing can happen in yaml files, but actually a little worse I'd say. In bash all the "special" things are at least symbols. But in yaml there are more special cases. Imagine editing this kind of a list:

js_keywords:
- if
- else
- while
- break
- continue
- import
- from
- default
- class
- const
- var
- let
- new
- async
- function
- undefined
- null
- true
- false
- Nan
- Infinity

Three of those are not strings. Syntax highlighting can help (which is why I don't think its a real issue). But still "why are three not strings? Well ... just because". AKA there isn't a syntax pattern, there's just a hardcoded list of names that need to be memorized. What is actually challeging is, unless students start with a proper yaml tutorial, or see examples of quotes in the config, its not obvious that quotes will solve the problem (students think "true" behaves like "\"true\""). So even when they see true is highlighted funny, they don't really know what to do about it. I've seem some try stuff like \true.

Still doesn't mean yaml is bad, every language has edge cases.