It’s not just semantics. There is no way to “disable animations”. There is no code that says “animate this plug LED”, so it can’t be turned off. What happens instead is that the window is redrawn many times per second, which may appear different based on the internal state of the program. If the state changes (e.g. the plug voltage is changed), the next frame will appear differently than the previous frame. This creates the animation illusion.
If you’re asking how Rack’s graphics thread can affect Rack’s engine thread, sometimes the operating system assigns two threads to the same core because another process (your browser, DAW, etc) is currently using the other one. Additionally, your CPU’s L3 cache is shared on all cores, so both threads share its bandwidth. There are many other reasons two threads might not be truly parallel.