Fire extinguishers are another type of insurance policy. Would you buy a policy from an offshore company that wasn't regulated by your state insurance commissioner? That's just what you are doing when you buy BlazeCut. They say they don't need to be approved, since extinguishers in cars are not mandated.
I am a retired fireman, and worked back in the '60's and '70's, when car fires, especially older VW's, were common. You will be struck stupid on your first car fire. It's always smart to have a portable extinguisher, but smart money is on the best fixed system you can afford. With a portable extinguisher, you will need to do several things quickly: undo your extinguisher, put on your gloves, release the engine lid, lift the lid, activate the extinguisher at the base of the fire. Does it sound easy?
I probably had to put out 10 car fires before I became proficient. I'm not a slow learner, but adrenalin surges don't help us learn.
All the US-made fixed systems I have seen (BlazeCut was made in the Czech Republic when I checked last) have UL Lab and NFPA approval. Save up for the right system, and hope you never need it. Every owner should also do a periodic safety inspection, using your sight, smell, touch, and hearing senses. Look for cracked or worn spots on your fuel lines, squeeze the rubber lines to ensure that they are still pliable, smell for gasoline, and listen for odd noises with the engine at idle that can indicate a loose fitting.
I don't even think BlazeCut is a halfway measure, since the owner will count on it to do the job of fire suppression, and it may fail.