Try to capture slo-mo video outdoor or use DC lighting and there wont be any flickering.
The flickering is because the fluorescent lights use AC. Our AC switches its polarity 50 times per second (50 Hz). There is flickering, its just it is too fast to be seen by our eyes.
So when you capture slo-mo video at, let say 120 fps or 240 fps, thats faster than the frequency of AC waves, hence the flickering can be seen.