I think that statement is way too strong and obviously not true of businesses. It might be true if hobbyist websites where the creator is personally more interested on the server side but it's definitely not true of professional websites.
Professional websites that have enough of a budget to care about the server side will absolutely care about the client side and will track usage. If 10% fewer people used the website, the analytics would show that and there would be a fire drill.
What I can agree with on the author is more of a nuanced point. Client side problems are a lot harder and have a very long tail due to unique client configurations (OS, browser, extensions, physical hardware). So with thousands of combinations, you end up with some wild and rare issues. It becomes hard to chase all of them down and some you just have to ignore.
This can lead to it feeling like websites don't care about client side but it just shows client side is hard.