I'm not trying to justify anything. I'm just trying to suggest you're labouring under a misapprehension. And this has nothing to do with security. I'm guessing you've heard the (good) advice that md5 is not a secure hashing function for, say, storing passwords, and then promptly joined the 'md5 is bad for all the things' cargo cult.

So while you're correct about the two images on that blog, the only reason why you'd get a clash is because the author of that blog post spent ~15 hours on an AWS GPU instance to generate the correct prefixes which, when appended to those files, results in a clash.

So, I guess if you are in the habit of grabbing random files from your hdd, loading them on to an AWS GPU instance for 15 hours (per file) and generating hash collisions, then yeah, don't use fdupes.