unclear what this means here: Before $$ occurs

[correabuscar]

in this footnote: https://veykril.github.io/tlborm/decl-macros/minutiae/metavar-expr.html#tt-$

Before $$ occurs, users must resort to a tricky and not so well-known hack to declare nested macros with repetitions via using $tt like this.

Sounds like what is actually meant here is something like "before $$ was added as a feature ... ", or so, but not "before you can actually use $$ in that code shown above or your code".

Maybe someone can phrase it better, assuming I understood it correctly?

[Veykril]

That is phrased weirdly indeed, you are correct with your assumption of what is meant here