If you really want petitions, start a real petition. The website is for something much more informal and you shouldn't get depressed just because 4chan isn't setting policy now.
High-signatory-count means a high level of engagement, among people who were exposed to the message. number of signatures / number of users on the site, or some other measure along those lines.
And the response largely ignored the thrust of the petition. I.e. that the side effects of prohibition are worse than the effects of the substance.
You also skipped over the part where the response given was bullshit boilerplate and amounted to ignoring it.
At that point, why have petitions at all? It's just handwaving.