Using symbols is Thread Safe in Rails ?

Hi,

     I read a good article about Thread Safe in Rails. Here is  the [link](https://bearmetal.eu/theden/how-do-i-know-whether-my-rails-app-is-thread-safe-or-not/).
             One thing that I couldn't wrap my head is  his example about Class instance variables. My doubt is how  just using a symbol becomes thread safe that block of code. Please let me know your thoughts.