I only know this word from context. And it is actually written with "k". So what does it mean? (Context/my reasoning: If you have to prove that you are a smart programmer you can't stay away from learning Rust. Hence it is kryptonite.)

Kryptonite isn't something you can't stay away from. It is something that makes you weak. As it did to Superman in the movie. An Achilles' heel.

Merriam-Webster's definition: https://www.merriam-webster.com/dictionary/kryptonite

You're looking for something like honey that no one can resist - kryptonite is Superman's weakness; he avoids it at all costs.