Nevermind the fact it crashes, how have they managed to get the 3D splash screen to display properly? That would at least mean the cart is being seen as a 3DS cart before boots.
They've just placed the
game's PCB in the flashcart's case. Easy.
You can see in the video that the PCB is red, unlike those of retail games.
The PCB seen on the screen is simply a large-scale replica of an actual cartridge. Why is it large-scale? To make modifications easy - it's a project board for christ's sake.
It works on a simple principle that if the hardware is 100% the same, the console won't know any better. Such a PCB is 100% capable of mimicing any game that has a cartridge that mirrors the build of the project board. It could be extended with sufficient switches that would allow mimicing numerous cartridges.
The games are loaded by re-flashing the on-board FLASH chip.
It's been done with the GameBoy and the GameBoy Colour. If they integrate the flasher and somewhat automate the process, as in, flash from an SD card then the whole contraption may be to some extent portable. Question is, will it fit in a cartridge case? I doubt it - most likely it'll be funnel-shaped like a PASSME, that is, if it will ever be released.
It's theoretically possible though.