MRI-4 thread scheduling vs Timeout.timeout

Ruby
1

I recently learned about timeout (forum does not allow me to link to github).
I read into its source code - it uses Ractor features, Fiber features and Thread features – the code is too clever for me to understand.

I have had the assumption that MRI-4 (ruby-4.0.1) does not preempt threads. Here is an example code that underpins my mental modell

def cpu_load()
  i=0
  while
    Math.sqrt(i)
    puts "t=#{Thread.current} i=#{i}" if i%400_000==0
    i += 1
  end
end

thread_list=[]
thread_list << Thread.new() do
  cpu_load
end
thread_list << Thread.new() do
  cpu_load
end

thread_list.each do
  _1.join
end

If you run the example, MRI-4 never switches the currently executing thread. So how does Timeout.timeout achieve to get scheduled?