I used to use a 200mm lens with a reversed 50mm on a Nikon D70, and the focus is difficult. It sounds like the photographer here is using a sort of "spray and pray" technique and using focus-stacking software later.
You actually don't need the telephoto, though. You can get a reversing ring that mounts a prime lens directly to the camera. I took this using a 50mm lens on a full frame body, no focus stacking: https://c1.staticflickr.com/1/498/19387253482_cfdb183b3a_c.j...
If you use a shorter focal length lens, you'll get a higher magnification. I've used a 28mm in the past but wasn't very happy with the image quality.
You actually don't need the telephoto, though. You can get a reversing ring that mounts a prime lens directly to the camera. I took this using a 50mm lens on a full frame body, no focus stacking: https://c1.staticflickr.com/1/498/19387253482_cfdb183b3a_c.j...
If you use a shorter focal length lens, you'll get a higher magnification. I've used a 28mm in the past but wasn't very happy with the image quality.