Right. And if the original site is down, the site will appear down even if accessed via a proxy. Saying the proxy is "down" is a misnomer in this case; the proxy itself is probably not down. However, using a proxy still won't enable users to reach a website that is down.
Neither the parent comment nor the torrentfreak piece are referring to mirrors.
Fortunately, Linux ISOs are generally available through other means. That's why this is on HN, right? To alert people to modify their Puppet configs to pull the ISO via direct download?
The parent poster is trying to highlight the 0.01% of PirateBay that is legitimate, so that the other 99.9% of "sharing" can be somehow legitimized by association.
Alternatively, they are trying to make a snarky suggestion that this isn't HN material.