I understand such blog spam is yet another plug for another thing where you need to swipe your card at some point for some questionable benefit. At this rate I would have thought that we are able to smell the crap as consumers, but there seems to be enough people who are willing to experiment with that.

For those who really want to "learn math" as autodidacts, nothing comes close to the Open University textbooks that are freely available in your libraries and also with some clever searching online. That material is refined over decades to support the autodidact use case.

The post you're complaining about is someone's personal blog, which appears to go back at least a decade.

Since you created an account just to complain about this, do you have a rule for what people can or can't blog about? Do you only accept posts about OSS and free materials?