That might be the "most effective" way to keep those comments off HN, but it's not the "best". Remember what HN actually is. This isn't a site that only exists to champion free-content-for-everyone-at-all-times-no-matter-what. Links are submitted to distribute news and foster good discussion.
The goal of this move isn't "stop people from talking about paywalls at all times". The goal here is to increase the average quality of discussion that takes place, when a paywall article is submitted. Different things.
I think that the content is at least a constructive contribution, even though I'm not sure that I agree with it; but is anything useful really conveyed by the '-1' at the beginning? (I don't know how voting works, but I doubt that "Tell HN" posts by admins are materially affected by up- or down-votes; but, even if they are, what's the point of advertising, rather than just explaining, your vote?)
The best way to keep these "off-topic" comments off HN is to make the sacrifice and stop rewarding those sites with traffic.