At the end of that article, he says the lens was too big to deal with, so he sold it, but I suspect the real issue is that it wasn't long enough for most bird and wildlife situations. Notice that his test shots are of buildings and ships, not wildlife. If he was getting the stunning MF nature shots he wanted, he'd probably find a way to transport the lens.
My FD 600/4.5 is a lighter weight lens (just under 8.5 lbs.) than the 600/4 for the Pentax 67 (the article says 17 lbs.), and still, I often need a 1.4X extender, and wish I had more (the 2X costs too much in chromatic aberration for my taste). Art Morris used the FD 800/5.6 L before he went autofocus and then digital, and he's a far better birder than I am, so he knows where the birds are and how to get close to them, and has spent many more hours in the field shooting. Now, with better optics, he sometimes uses a 600/4.0 and stacks two 2X extenders on it.
I've got a few good bird photos made with a 300mm lens or even a 200mm lens, and maybe a couple with the 500/5.5 Tele-Xenar that I adapted for my Bronica, and even a few with the 360/5.5 Tele-Xenar and a 6x7 back on my Technika, but the vast majority in the keeper file are with the 400/4.5 and 600/4.5 on 35mm.