TL;DR: for pretty much all users this scenario is very unlikely and much lower risk than, say, some devious jerk going low-tech and throwing a bunch of marbles in front of your Ninebot.
As someone who spends some time working in computer security...
From the researcher's video, it looks like the attack relies on a modified Ninebot to succeed. In other words, a hacker would need access to your paired phone and Ninebot (or the ability to pair their own phone to your Ninebot)--and would then need to reprogram the Ninebot with special "attackable" firmware.
That's a much different scenario than some random stranger being able to force your machine to turn off at will. [And again, a bucket full of marbles, or a bucket full of slippery goo, or maybe even a low-power EMP...would all be simpler and more effective troublemaking.]
This is a common issue shared by many, many millions of devices...if someone has physical access to modify a device (either by reprogramming its hardware or by modifying some of its parts) then they can make the device do their bidding.
In this particular case, Segway could issue a firmware update which required all future firmware to be digital-signature-authenticated. For reference, desktop operating systems made that exact move over the last two decades. [There are also nifty things that can be done with crypto-authentication microchips...a topic for another day perhaps.]
Also, please note that everything I just said is a gross oversimplification of the hardware/software security issues inherent in any wirelessly-connected device.
Chris