I've always wondered if a car is still insurable after one loads custom firmware.
There are separate modules that do well-defined tasks and only a few of them would ever be relevant in an insurance/accident scenario (engine/transmission controller, power steering and maybe ABS).
What he's done here is equivalent to installing an aftermarket head unit, something people have been doing for decades. But even editing firmware (or rather the data tables in there - also called "maps") from more "spicy" controllers such as the engine management controller is routinely done by auto tuning shops and in practice the worst that happens is blowing up your engine by pushing it beyond its limits.
With that said, yes, if you happen to have a suitably common vehicle (e.g. Ford F150).
Zenec for example makes units that are able to display & control HVAC and parking radar on VAG Group (Volkswagen/Audi/Seat/Skoda) vehicles, pulling the data from the CAN bus.
In the authors case he’s reusing existing manufacturer-provided libraries that handle CAN message parsing but there’s no reason you couldn’t do it from scratch with enough effort.