AFAICT, the original use of a triangle to describe a relationship between ISO, aperture and shutter speed was by Bryan Peterson in lectures he gave. He then published it in "Understanding Exposure" with the name "photographic triangle".
Peterson seems to have known that the triangle didn't actually fully describe exposure. His name was thus somewhat more appropriate than the term "exposure triangle".
Peterson was an available light roll film photographer who specialized in landscapes and streetscapes in the second half of the 20th century. For him, the scene luminance was a constant, not a variable subject to his control. He had this in common with the vast majority of new camera owners, so his approach to operating his camera worked well for them.
The three elements of the photographic triangle were the three main decisions a photographer made to affect an exposure. In Peterson's typical case, the first choice a photographer made was to select a film speed, and load a roll of that film. From then until the roll was used up, the only exposure-related decisions the photographer had to make were how to set the aperture and shutter in light of the available fixed ISO (ASA or DIN).
Peterson's approach was a reasonably good one for available light roll film shooters limited to standard developing. It was not so good for studio photographer who controlled their own light levels, or shooters like Ansel Adams who made exposure decisions in part based on expectations of non-standard developing in a process more oriented to maximum image quality than was Peterson's approach. In the digital era, Peterson's approach, with minor tweaking, still works for JPEG shooters who are concerned more with convenience than maximum IQ.
Given that film speed was fixed, and development was assumed to be standard processing, usually by a third-party lab, a given exposure decision produced images of the same brightness. The distinction between exposure and brightness was lost to the consciousnesses of most photographers, and still seems to be obscured from many today.
While Peterson's approach was a reasonable one for natural light roll film photographers using standard processing (which was the vast majority of shooters) it is decidedly suboptimal for digital RAW shooters. With Peterson, the primary goal of exposure decisions was to produce an image of the correct lightness. Shutter and aperture would be adjusted reciprocally depending on a choice between DoF or motion blur having primacy of importance, but the specific combination of the two was subject to the overriding choice of ISO. WIth digital RAW, it is trivially easy to adjust image lightness after capture, and there is usually no need to retain the same ISO from shot to shot. Thus the most important pre-exposure decisions are those that affect image attributes that cannot easily be adjusted after capture: highlight clipping, shot noise, DoF, and blur from motion, aberrations and diffraction. All of these attributes are results of exposure (which doesn't include ISO) , or of aperture or shutter. With RAW, ISO selection properly becomes subordinate to exposure optimization.
