This article is written for web developers. I’m not sure who you think you are addressing with this comment.
In any case - the argument is a weak one. To the extent people make the mistake you allege they can make it with classed div and span tags as well and I’ve seen this in practice.
But semantic purposes are not all possible purposes.
You could say the same about divs. I’ve seen pages where everything is a div. No paragraphs, no headings, no tables, no lists, just divs.
My point is that if you are using <div class=my-element> you don't have to change your .my-element CSS selector or JS selection code to improve your code to <p class=my-element>. If you are using <my-element> it is much more work to change your selectors and now you have two ways of doing things depending on if you are using a native semantic element or a div (either a tag selector or class selector). You have made your styling code depend on your element choice which makes it harder to change.
It's not. For semantic purposes <my-element> is the same as <div class=my-element>. So on the surface they are equivalent.
But if you are in the habit of using custom elements then you will likely continue to use them even when a more useful element is available so <my-aside> rather than <aside class=my-aside> so in practice it is probably worse even if theoretically identical.
Basically divs with classes provide no semantic information but create a good pattern for using semantic elements when they fit. Using custom elements provides no semantic information and makes using semantic elements look different and unusual.