I don’t know about it causing catastrophic damage. I’ve clamped for a while now, and haven’t really experienced any side effects other that base irritation and a slight ache. I think you guys are referring to clamping behind the balls, as it can be dangerous. Since you have a lot of sensitive tissues in the testicles. You can easily cut off the circulation there, and that would be serious.
Clamping is an advanced girth exercise, and dangerous. Just follow the guide and keep the clamp on 5-10 minutes a set. Then I think you’ll be fine. Besides, I don’t think you’d cut the circulation of blood in your dick off. That’d be kinda hard.
Another thing, when you clamp don’t do it mindlessly. That’s where all the damage can stem from. To stop irritation, put some lube on your penis and go for it. Just don’t do the Behind the balls clamp.