I suppose I can make my argument here for why I consider this to fall more into the console space with only some bleed over into the handheld space. Something of an 80% home console and 20% handheld.
1) First off, size. The Switch unit is a bit large for a handheld traditionally. The biggest and most distinct feature of a handheld obviously is the high convenience of portability due to being able to slip right into a pocket or a pouch in a handbag, satchel, or purse. The Switch really can't in most cases. There are extremes: the Game Gear was oversized (also failed btw) and some people could fit the Switch into those things. But for most, the Switch unit is large. And that's before you factor in the Joy-Cons, which are mandatory to use it at all. Not only do those make it even larger, they also have full analogue sticks that would give you a real issue trying to pocket this or put it in a standard satchel. This is something you will need a more substantial case or to just carry. Cause it is mostly meant to be portable for short contextual periods or taken to a specific place and setup (airline tray, party table, etc). Not be available at a moments notice at any time anywhere like handheld.
2) Second, it is (apparently) actively cooled. This also ties into number one cause this seriously changes the convenience of portability. It means this thing will run at a higher temperature that will make pocketing it unwise for comfort's sake if nothing else, not to mention you would want it to cool and breath. Again, need a carrying case. Also, the vents and empty(ish) space around the fan area to facilitate cooling compromises the structure. It will still be plenty strong, but you would definitely not want to subject it to the pressures of a pocket or overly full purse/satchel. Unlike a handheld which is traditionally pretty dang dense and thus, again, convenient to pocket since it can take those stresses exceptionally well.
3) Third, handhelds have physical inputs integrated into the device and are not reliant on any peripherals. It's simple: handhelds have controls built in, consoles rely on controllers. The Switch unit is not capable of any use as a system to our knowledge without a controller, either a Joy-Con or the Pro. This runs directly contrary to handhelds which are built to have literally everything needed to game integrated into a single piece of hardware with no need for aditional physical addons. Consoles need controllers, handhelds do not. And the Switch needs controllers.
4) Fourth, a major distinguishing feature of home consoles is integrated same-system local multiplayer capability. In short, since handhelds have physical inputs entirely puilt into the device, they cannot support true same-system local multiplayer. That is, any multiplayer that require simultaneous inputs. This is the whole reason why golf games found a big audience on handhelds for quite a while, it's one of only a few ways to have any true local multiplayer because of this inherent design limitation of handhelds. The Switch, obviously, relies on controllers. And so we see already, it has support for true same-system local multiplayer. That is, splitscreen. That's a major distinguishing feature of consoles vs handhelds.
For those reasons, I regard this as primarily a home console with mild overlap of handhelds. People look at this and say "oh it's a handheld that has evolved to somewhat overlap with consoles" when in reality, it is the evolution of this:
That is, it is aiming to be - from launch - a home console that is easily and conveniently portable with the option to play on the go. And I would say *that* is the word Nintendo is really latching on to: convenience. It's literally in every aspect of the design. The Joy-Cons allowing for local multiplayer with no new controllers, the use of cartridges to allow playing off the physical media for modern games to avoid dealing with installs and get back to plug and play, the size allowing for portability, the battery and integrated screen allowing for playing if there is no TV. All centered on the original console selling point of convenience. Will it bridge handheld and consoles for them? Probably, once they can get a smaller and cheaper version out, it's a bit big and pricey now. But from a design philosophy perspective, I see a lot of home console design elements here that run contrary to handheld design philosophy. Basically, just cause you can pick up the core unit and it has a screen doesn't mean it is a handheld, there's more to it than that.
Edit: Oh, and using mobile tech has nothing to do with a console being handheld or not. Some mobile tech is used in the Xbone and PS4 as well. Where the tech comes from doesn't matter, it is the design that matters.
Edit 2: I guess I could say that the Switch is the same thing laptops are to computers, an in between between a true computer and a handheld computer. Well, back in the day when "handheld computers" weren't just phones :P . Basically, a device built to give you most if not all functions of a computer in a more portable form while the handheld computers (and now phones and tablets) are their own beast, with the portability aspect being the only thing they kinda share.