What really matters is: Is the final image what you are trying at achieve? Here is a test I did with my Sony A1 shot with the Sony 200-600mm G, As cameras progress, they improve on the difference between the lowest ISO and the highest. There is a world of difference between the images in my test here and a test I did 10 years ago with my Sony a65. In the film days, your max ISO was about 1000 and it was very grainy and weak color. I would not have been able to get a star shot even close to the one show here.

ISO is separate from exposure. So is light value (LV), the brightness of the scene.

Below is another example. The light value of 15 represents the brightness of a daylight scene. It's what the meter (incident or reflective) would measure to determine the brightness of the scene brightness. At ISO 100, it also represents the exposure value (EV) needed to capture the image properly, assuming there is no ND filter over the lens.

That exposure value can be arrived at via any number of different aperture/shutter speed combinations.

Neither the ISO nor the light value are related to exposure. But to get a reasonable image the three values (ISO, aperture and shutter speed) must be compatible with the light value.

Bill, wonderful capture, and I assume, processing. Yes, ISO-100 is a dream outside the scope of most photography situations. Also, different camera brands approach noise and ISO differently. I put together some posts in the past showing processed images at (then) rather high ISO values. As mentioned above, I also sought always to maximize the light reaching the sensor before addressing the ISO needed. Or, when at ISO-5000, I was at the 'max' for that camera and needed to address the aperture and shutter to get an image to then apply ISO-5000 against.

Shooting at high ISOs (ISO-5000)

The 'bottom' of this 5000 link takes the reader to 3200 and 4000 examples too. Some of the images have been reprocessed over the years, breaking the UHH to Flickr link for image display. But, enough of the images remain 'as is' to demonstrate what is possible at ISO-3200, and above, in low-light. The full-exposure details are on the Flickr pages.

I haven't read all the mudfight since earlier, but to your question about 'good exposure', in digital photography, that pertains to maximizing the light hitting the sensor. That is done in one of two ways, or a combination of the two: the duration of the shutter opening and the size of the aperture. Nothing about ISO is involved. ISO comes after the light hits the sensor.

Digital photographers should always seek the minimum shutterspeed needed for the composition and the widest aperture appropriate for the composition. Don't shoot at 1/2000 sec, when all you really need is 1/500 sec. Don't shoot at f/11 when the subject is covered perfectly by f/6.3. Artistic intent goes a long way in determining aperture and shutterspeed, but just blindly selecting values, outside of direct sunlight, lead to issues in digital photography. And the bottom line: maximize the light hitting the sensor to minimize the ISO 'gain' needed for the resulting image. That is the 'nutshell' of 'good exposure' in digital photography.

The triangle nonsense may be useful for the entry-level, or state-school coursework, but anyone with a firm understanding of digital photography and the underlying technology of how ISO is implemented in digital photography, they will worry about their shutterspeed and aperture. They understand the performance of their camera at every ISO setting, and will know the inflection point where the camera's noise performance goes from just fine to problematic. They will always seek to maximize the light hitting the sensor, for a given composition, to hold the ISO to the lowest necessary value.

The triangle implies the three points are equal. They are not. Shutterspeed and aperture are equal. ISO was born out back, not in the barn, but behind it.