I thought his point was that people who like it tend to be newbies to web development who are not able to make an informed critical judgment about its merits be

I thought his point was that people who like it tend to be newbies to web development who are not able to make an informed critical judgment about its merits because they don't know enough of HTML to begin with - they learn HAML by seeing its output without understanding the HTML that underlies it. But I agree that this seems like a weak argument. I can't imagine working in web development without understanding HTML.