How's this supposed to make any difference about YAML (or this Restricted YAML) being used for things it's not supposed to be used for?
It's not a problem that $CI YAML configs have multiple ways to encode strings and need indentation. Yes, "no" issue sucks a bit, but it's not that huge, just a minor papercut. The giant root issue is that YAML is simply not an appropriate format for this kind of stuff. People are writing more and more complex programs (modern CI pipeline is a program*) in it, when it's not a [suitable] programming language.
And it's not suitable not because of whitespace. Syntax doesn't matter that much (unless you're a junior developer first time seeing APL - just kidding, of course), it's all about the semantics. YAML was never meant for writing programs, it doesn't have necessary constructs to support those (but people are inventing DSLs on top of it, and it looks and feels awful).
Similar logic applies to some orchestration systems, some IaaC stuff and other areas where YAML is not a great fit because systems either "outgrew" simple configurations to something else (like programs, but not always) or YAML was a bad fit from the very beginning. CI is just the most obvious example.
*) It made sense when CIs were simple, and those YAML files were configuration snippets rather than programs. Just like Makefile makes a lot of sense, when it's able to do all you need. But CI grew more and more complicated.