A 70-120 zoom at 90, a 90mm prime, and a 70mm prime cropped in to the same field of view should produce the same perspective projection. Try it sometime – take two photos from the same spot, one at 70 and one at 100 and then crop the 70 to the same field of view as the 100. It won’t be perfectly identical but those differences are due to lens imperfections, not something fundamental.

Not gp, but as an example of what they mean: take a photo at 70mm then at 120mm. You can crop the first photo to be have the same composition as the second, but the resolution will be lower.