Sharing it publicly is the author saying it does not actually work as he claims it does. You don't write a blog post on a method you created to make guaranteed money, you use it.

A couple years back there were some traders talking about trying to implement a really fast version of K-means for clustering stocks that are transiently correlated on some sub-reddit. I wonder if this guy is the one who started that thread.

Here's how this always goes: Someone thinks they have a killer algorithm and it works and makes them money...for a time. Then it suddenly doesn't work anymore and they lose more than they gained. It is not possible to beat the market over the long term unless you are trading on insider information, somehow manipulating markets, front-running orders, or have some other edge that is not universally available. There is no Python script that will simply print money for you year in and year out it doesn't matter what data you feed it or how smart your AI is.

Why is it impossible to beat the market over long term without insider information?

Because by definition in order to make money you are abusing a market inefficiency. If you use this inefficiency, you're effectively shrinking the inefficiency to closer to nothing every time its used.

I would argue that this scenario would actually be a positive for the author. If others start using the same methodology as him on a large enough scale and trading on the same data sets they would buy when he buys driving prices up.

On the other hand when he starts selling and others sell as well speed is a critical factor as the downward pressure will make the later movers the biggest losers.

It's just SEO.