The iPad lacks flexibility and as a result it is less generally useful than a "PC". Can we agree on that? I own a 12.9" iPad Pro, I love it, and I'm fully bought in with the pencil and the keyboard folio and all that shit. If I want to write or draw or browse the web or watch a video, I reach for my iPad first because it does all those things just as well as or far better than my laptop, and nothing would delight me more than to be able to toss my laptop in the trash (or whatever). But I can't!
Neither can my partner, who does most of her computing (largely consumption) on her iPad, but often has to interact with institutional websites that have poor compatibility. Neither can my mom, for whom I bought an iPad in 2012, which she still uses and loves, but not even the latest and greatest iPad can support her hobby genealogy research workflow, so I had to buy her an iMac, too.
The author's tone may strike some people as inflammatory or self-centered or whatever, I guess, but his essential point is right: the iPad's apps-first design and locked-down nature keep it from being a first-class general-purpose computing machine, especially for creative work. That it works for some people is great--we don't have to fight about that, nor should we. Obviously it is a crazy good form factor and fully capable for many real professional tasks and workflows. But it should not be recommended as a PC replacement for somebody who hasn't looked at all the angles, because it simply isn't a fully capable replacement, and that's what the author is talking about. If anything, I think the author's point is made more narrowly than it could be.