oh yes, subdomains gone wild have to do with SEO, but not in the way outlined by the author.

duplicate (sub)domains resulting in duplicate sites with duplicate pages mostly have a negative impact on the performance of a webproperty in the SERPs.

why?

a link to www1.example.com does not automatically count as a vote for www.example.com, a link to www.example.com does not count as a vote for www1.example.com, that means they now how two websites both with one vote, instead of one websites with two votes.

additionally, you have two websites which are in competition to each other, and each of these websites has duplicated webpages which are in competition to each other. both websites and webpages usually perform poorer - if google has a doubt which of these duplicated pages on these duplicate sites is the best page to point the user to (said that, google is pretty good in stripping doubt out of the equation for subdomain duplicate pages issues)

if you have a webproperty with a "subdomains gone wild" issue, it is best practice to canonical (either via the canonical tag or via HTTP 301 redirect) them to one (sub)domain. it almost ever (depending on how big the issues was) results in a better performance of the canonicalized webproperty - it definitely helps the site on the (organic) linkbuilding front.

there are thousand reason why a webproperty can have a subdomain gone wild issue (it was (once upon a time) even a common black hat practice of spamming google with subdomain gone wild duplicate web-properties of the competition sites if possible (and it's possible with sites with a wildcard subdomain setting (i.e. wildcard.w3schools.com))

but in most cases "subdomains gone wild" does not have a positive impact on the SERPs. (blocking results is not one of this cases.)

but yeah, what the real issues with w3school? why does this sh/t rank so well?

well first of all it should be said: "you are not statistically significant" just because you (in this case we, the HN readers) are not happy with w3school does not mean the average searcher (searching for HTML web dev stuff) is not happy with it. they are happy (hey, they don't know better) and they use it like crazy. they are happy with what they find. as google is measuring SEPR "long click" very effectively they know exactly how well the average search users uses a page/site. if all users would click back immediately, would not stay long on the page, w3school would not be so dominant as it currently is.

secondly: links - i just did some backlinks check ons some obscure w3school URLs, they all have links. now you might say: oh they buy links... does not look like it, they have links from old forums, new forums, blogs, .edu domains, ...

and: where is the competition? mdn does a great job, but it is a resource for developers, for people who know what they are doing. w3school is a great resource for people who do not know what they are doing or what they are looking for - and w3school has a special page for every single thing / tag that we don't even bother to mention anymore (it's outdated, it's old, some say ugly, but it's there and has a unique description text, a unique example on the page) - that means they have a page for most of the people using the internet interested in HTML, people which think of HTML as a "programming language" - for them w3school is the perfect product, no competition in sight.

what's the solution:

in the xoogler book "I'm feeling lucky" the author describes a case where a wrong product keeps ocurring again and again for popular product searches. they tuned the algorithm again and again, the results got better and better, but the product showed up again and again. the engineers didn't know what to do. one day, it was gone. what happened: one engineer just bought the product. it wasn't listed in the store anymore. the issues was fixed.

fazit: somebody who cares about the internet (w3, mozilla, google, adobe, microsoft, opera, ..) and profits form good, well structured HTML and working javascript should just buy it (can't be that expensive).