War is unpredictable. Ukraine is outmatched but defenders advantage goes a long way. Total defeat of Ukraine's military in the field wouldn't be achievable in 72 hours but it was still possible for a surrender to have occurred in that timespan if the chips landed the right way. While they definitely do exaggerate their predictions for several strategic reasons (budgetary, propaganda, cointel) the element of simply preparing for the worst case scenario is probably still the primary reason and no analyst gets gets their name dragged in the mud for having urged too much caution.