It is specifically saying the same problem exists in JSON/YAML/TOML, etc, which is that all these configuration languages don't have any real means of abstraction, and ultimately aren't expressive enough do to the job we require of them.
as soon as you are templating config files with other configs, I agree, I have sorely felt this limitation with helm charts
There is no single cure-all, of course, but surely we should be relying on computers to do much of the heavy lifting when it comes to validation and verification of these files, not just as linters after the fact but in realtime while we're editing, and with some sort of knowledge (even if derived programmatically) of what is right and what is wrong so we no longer have to worry about simple footguns.
But when working with real programming languages it is completely different, you can take semantic information from the current code, and you can have things like types to give you safety.
But anytime someone suggests TOML I have to double check to be sure they are serious because the TOML syntax for anything more complicated than single-layer maps is mind-bogglingly confusing to me. This is not a serious alternative to YAML.
Trying that on Google gets me https://en.wikipedia.org/wiki/Sayre%27s_law. Is about right. :D
Don't want to sound too harsh, but to me HCL is even worse than plain YAML.
By expressiveness, HCL is somewhat similar to Ansible-flavoured YAML - in both you need to use magic keywords to create any kind of abstraction (e.g. a loop).
HCL is worse than regular YAML because there's only one "true" parser for it, that is official Hashicorp's HCL parser. So if you are locked into Golang ecosystem, then sure it can work for you, otherwise you are out of luck.
There are a couple of tools that convert HCL into JSON, I tried both, they somewhat work, but in the end of the day it's a big hack. At that point I just gave up on using HCL and started using something else that generates JSON.
Hope you find some configuration layer that fits your users more than HCL or YAML.