The reason seatbelts release tension is a compromise in safety made for the average driver (i.e lowest common denominator). The engineers probably thought they would compromise on safety because otherwise most people are not capable or willing to operate a harness. Aircraft and high performance automobiles use a design that would make perfect sense to a nine year old.
And more importantly, this misses the point of the original article, in which the author didn't understand how seat belts work at all.
If you do install a well-designed cage just so you can safely use a harness, now you also need to wear a helmet. A cage without a helmet is begging for a bad head injury, because your head bounces around a little during an accident, and those bars are hefty, rigid, immobile objects mere inches away from your skull.
Even in a fairly low-speed incident that wouldn't cause other injuries, the bars would put a dent in your naked skull. You're also going to want a neck restraint system for that helmet (it's like a collar under the harness with little straps to the helmet, so that it can't move far), in part because of the added head weight (momentum) from the helmet and how rigidly your torso is locked in place, to prevent neck injuries.
Now that you have all those parts down: the combination of the harness locking down your torso, the neck restraint, and the limited window of visibility from the helmet means you've lost a good chunk of your peripheral vision and can't turn your head/neck to check things either. With your vision more or less locked in straight forward, you can't see all the things you really need to see to drive in normal street conditions safely. The limited vision works fine on a racetrack under racing conditions because everyone's trained the same way and operating in a sanitized environment, with no "intersections" or cross traffic, you have flag workers and/or radios to warn you of some things you can't see, etc.
The bottom line is that both street and race car safety systems are a whole complex of things that are engineered together in concert. They're well-tuned to the appropriate environment, and foolishly mixing ideas from the two generally makes you less safe, not more.
Random context video showing a race crash from the inside and outside perspective. Loss of brakes at the end of a high speed straight at COTA (he was still moving at 136 mph at the wall impact, after all the attempts to slow it down). The driver walked away fine, thanks to all the safety gear that the cockpit video shows off nicely near the end: https://www.youtube.com/watch?v=dQitOyEyRd0 .
So maybe, and this is a guess, airbags are not activated in lateral impacts like this one as they could make this escape more difficult than the damage they are preventing.
1. In a regular car, most accidents are at relatively low speed, and in those at high speed are generally front collisions (for at least one of the cars), so cars are designed in a way that the front and back are crumple zones to absorb the shock and limit energy transfer to the passengers. So after protecting passengers from flying out of (or within) the car with the belt, airbags are there to prevent whiplash from front/rear hits.
2. In a racing car, impacts are usually at high speed, and in basically any direction. Accelerations and decelerations will be significantly more brutal, and particularly for rally cars, vehicle rollover is really common. Hence the rigid cage + harness to reduce direct damage to the driver in those events. But even so, since the helmet's weight and the rigidity of the vehicle can make whiplash and basilar skull fractures, there are also helmet restraints (like HANS[0]) to reduce this risk.