It actually explains what alpha is. The line above its first occurrence it says: "In the case of polynomials". So alpha are the coefficients of the polynomial.
What you are describing is how to remove _access_ to the secret. The issue here is, that the former employee still has _knowledge_ of the secret. Hence you have to rotate the secret.
CoreOS is a Linux distribution which was originally designed to _run_ Docker. So it's not really an alternative to Docker.
I guess they meant Rocket (https://github.com/coreos/rocket). Rocket is an app container runtime similar to Docker, which is developed by the CoreOS group.
https://signalsandthreads.com/what-is-an-operating-system/