Backwards compatibility is given in HTML by the doctype. I think you might also really be talking to forwards compatibility in addition - "what I would write in an HTML 4 document will (almost) always work the same in an HTML 5 document".
I don't think what the author is talking about is necessarily against either. HTML5 can continue its evolutions while "HTML6" (or something completely separate) exists alongside the traditional DOM, like the <canvas> example they reference or like WASM exists alongside JS. From this perspective, the article is about why it's a worthwhile time to make the new thing rather than why the new thing won't also have baggage in 20 years or why we should just throw everything current out the window as part of supporting the new thing.
I don't think what the author is talking about is necessarily against either. HTML5 can continue its evolutions while "HTML6" (or something completely separate) exists alongside the traditional DOM, like the <canvas> example they reference or like WASM exists alongside JS. From this perspective, the article is about why it's a worthwhile time to make the new thing rather than why the new thing won't also have baggage in 20 years or why we should just throw everything current out the window as part of supporting the new thing.