I don't see the point of this article, it's just common sense. If you plan which parts of the DOM are replaced using HTMX, never trust user input. You'll be fine. I use Golang with HTMX and it's amazingly productive.
I think the point is that htmx is in no way special. It's the same old category of vulnerability. It's why we have execvpe. It's why we have mysql_real_escape_string_no_really_we_mean_it_this_time().
I remember a similar point being made about LLM output. Now, I'm anything but a LLM fanboy, but if you pipe unknown text into a system interface that's squarely on you.
I think the point being made that you're replying to is that it has nothing to do with htmx. It isn't htmx that's not playing well with content security policy.