It's not a PDA because: It lacks a calendar (although you could write one for it). It lacks a contact database (although you could write one for it). You can write software for it without using another computer. It has a full-size keyboard. It doesn't fit in your pocket, even if you get a Steve Jobs special shirt with extra-large pocket.
So, why is it not a laptop? It did most of what any contemporary computer did at that time, and it fit on your lap comfortably. If someone built a laptop today that was lightweight and consumed very little power, but it was only as good as a 5-year-old computer, would you say it's not a laptop? Because that's basically what Tandy accomplished with the Model 100. (Actually it was probably more like 1-2 years behind state of the art personal computers of the day.)
Or maybe you're hung up because you think the article is talking about this being a replacement for modern laptops -- that would be a very ludicrous thing for anyone to suggest, but I suppose we've gotten crazier articles than that before.