Aspect ratio by itself tells you very little about the theatrical viewing experience. What really matters is the "angular image size," i.e. the horizontal and vertical angle that the image fills in your field of view. This is determined by the aspect ratio of the image, but also critically the size of the screen and the distance from the screen to the viewer.

1.43 IMAX screens are really really big and fill more of your field of view than most other cinema screens. You can't just look at the 1.43 aspect ratio and say "it's skinnier than 2.39 scope" or "it's wider than 2.39 anamorphic." It would be more accurate to say "in your field of view, it's both wider and taller than most theater's 2.39 anamorphic projection."

This is also why pleas for the 1.43 aspect ratio on home video are slightly misguided. It's not the shape of the 1.43 IMAX image that's so compelling. It's the angular image size! And you probably are not going to view an IMAX film at home with the intended image size. (To be clear, I think it would still be interesting to see that 1.43 image at home, and in VR it's reportedly an excellent experience.)