What the author of this blog is doing is essentially server side react, but without the VDOM diffing. If there was DOM based diffing, then the only things that would change are a bunch of class attributes that affect which step becomes visible.

To avoid the cost of updating the entire page, htmx only fetches a parent element and all of its children, but this runs into the problem that you must choose the common parent element for all the elements you want to update.

So the author reaches the conclusion that htmx is not meant to be used for SPA style apps. It's meant to add a little bit of interactivity to otherwise static HTML.