What made the iPhone revolutionary was the input method (and of course all the UI built on top of that).
I kinda glazed over after that intro.
Thus I kinda inferred the OP meant "... which device crossed the threshold into mainstream awareness". So your "glazed" dismissal seems a little harsh.
That said, I think you're right to make note of the Hiptop (TIL!), and I agree that it was the touchscreen (albeit also not truly firat of its kind either) that was a transformative experience for the masses. I'll never forget the first time I interacted with an iPhone.
The hiptop unquestionably crossed the threshold into mainstream awareness at the time. It's just been forgotten because the iPhone overshadowed it.
On a personal note, I had a few generations of Hiptop before I switched to the 1st gen iPhone. Yes, my first experience with the iPhone was memorable... but still not as memorable as my first Hiptop. "You mean I have the internet everywhere??" That was incredible in 2002. We just take it for granted now.
Or maybe because it never successfully left US and reached the rest of the world.
The connected devices at the time were all still struggling with the early adopter chicken-and-egg issues of an emerging network of services. There weren't "apps" and web sites had not centralized themselves. And cost, speed and service coverage remained very limited. You couldn't justify a data plan just for Mapquest when you could print it out at home. Most kids, myself included, wouldn't be getting any phone for a few years yet. The mobile phone's purpose in the 2000's was served well with what was in feature phones: call, text, Snake, maybe email.
In contrast, the iPod, a rough contemporary with the Hiptop, addressed something more immediately compelling with a two-sided, integrated marketplace when paired with iTunes. There was a value proposition in that since not everyone was or wanted to be a savvy song pirate, and you could buy singles instead of albums. Internet speeds and access were ready for that use case.
Apple's success at the time, both with iPod and with iPhone, rested on timing and quality of integration, which returns again to that which the article alludes - we have a lot of early-adopter developer services, and some of these are in a position to be more like an iPod/iTunes. But I don't think the article goes deep enough in recognizing that even the iPhone was capitalizing on underlying infrastructure developments to channel them through a specific product and service mix.
Email was (and still is) the killer app, and it worked great. Instant messaging worked great - and at the time, Maxis (where I worked) ran on AOL IM the way people use Slack now. Danger's web browser actually worked through a proxy that chopped up pages and served them in a reduced format, which was effective in 2002 but was a liability by 2007. You could even edit your contacts online with full syncing!
The Hiptop was popular and "quality of the integration" was better than anything Apple had produced right up until the iPhone. However, by 2007 the data networks had gotten fast enough that the Hiptop's web browser was feeling a bit antiquated, and navigating web pages with a scroll wheel was cumbersome. The capacitive touchscreen was a major leapfrog, and coming up with all the UI behaviors to leverage that was a major feat - Apple deserves a lot of credit! But maybe not quite as much credit as the OP suggests.
https://en.wikipedia.org/wiki/Nokia_3510
What I could I do with 2003 Internet?
Download J2ME applications from Swisscom and Sunrise, including some crufty map applications based on cell position.
I knew a bunch of deaf kids around that time and they all were enthusiastic Sidekick users.
Was out well before the iPhone, sold Internationally, had installable 3rd party apps (proper apps, not just "web" apps that the 1st gen iPhone had).
Apple fanatics always seem to think Apple create the first smartphone as we know it.
"Symbian OS platform security model"
https://www.usenix.org/system/files/login/articles/73507-li....
While the USENIX paper is from 2010, the 3rd edition was released in 2005, 2 years ahead of iPhone.
And the now super fashionable Widgets were already a thing on Symbian, back in 2010.