There is no comparison. MM is an exact measurement that remains relatively equal across cameras. X zoom is simply what it sounds like. I have a 150-600mm lens. It is a 4x zoom. Yet a 10-100mm is a 10x zoom. A 10-200mm is a 20x zoom. Simple multiplication. Otherwise I wouldn’t understand it. 🙂
It also works for marketing. They can advertise a 5-200mm lens as having 40x zoom. Yet you barely have enough to shoot wildlife.
A prime 800mm lens has 0x zoom, because it doesn’t zoom.
Let me know if I can explain it farther.
Austin