The current trend started when people noticed that sam altman doesn't capitalize sentences, primarily in tweets.
In longer form, I do think people should feel bad for writing like this. Like you said, capitalization has genuine utility and, without them, it makes the blog post a nightmare to read.
That trend has been around for far longer than Sam Altman's fame has. I see it more as a way to prove that the content is "authentic" and "human", although I of course can't vouch on where the author here got their inspiration from.
To paraphrase a saucy joke I read on Twitter, caring about case makes the difference between helping your Uncle Jack off a horse and helping your uncle jack off a horse.
In longer form, I do think people should feel bad for writing like this. Like you said, capitalization has genuine utility and, without them, it makes the blog post a nightmare to read.
Case in point: even Sam doesn't write blog posts like this https://blog.samaltman.com