[<!--Hidden HTML comments-->[ in the old parser is treated the same as [[. This means that [<!--Hidden HTML comments-->[Link]] turns into [[Link]], which seems unlikely to be the wanted result.
[[ is how you open a link, and ]] is how you close it (e.g., in a help page) is rendered as a red link to the text in between. [<!--Hidden HTML comment-->[ is how you open a link, and ]] is how you close it should behave like a <nowiki/> tag and therefore produce plain text. (Yes, it'd be simpler [for me] to use nowiki tags, but not everyone knows about those.)