Problem is, the CSSOM models that more like this:
<rule selector="h1, h2">
<style>
<property name="color" value="red" />
</style>
</rule>
Perhaps your takeaway from this is "the CSSOM is bad" (and I don't necessarily disagree), but it's what the "mistake" is talking about.
I wouldn't say that, it's more that the CSSOM doesn't try to preserve comments, which is a perfectly reasonable thing to do. I think most uses cases for modifying CSS that care about comments (e.g. auto-formatters?) would need parsers that return the full CST anyway.