This is not a vulnerability. If someone can modify your environment variables or /etc/ld.so.conf, your system is already wholly, entirely and utterly compromised.
Hey, we agree that if someone can modify your env variables you have got problems ;) But, if you have valuable data on your system then you should have defense in depth so that your most important stuff (secrets, etc...) isn't stolen.
Sure, but that's not what your article is arguing. You literally have a heading "The Vulnerability". It's not a vulnerability, it's not an attack, it's just one option of what you can do after you're done exploiting your way into a system. Not even sure it's a particularly good option; modifying environment variables will mean that at least the target user is fully compromised. In turn, that will mean in pretty much all cases that the attacker is able to just transfer out any and all private keys. And note LD_PRELOAD is only applied when you start something; restarting a long-running process might in itself raise alarm bells or require re-unlocking keys. Much easier to directly take the keys from running process memory.