If I am understanding this correctly, previous studies didn’t account for existing fitness levels. Less-fit people need considerably more exercise to obtain the same level of CVD risk reduction.

So fitness at all ages is important.

the more modest 8%–9% reduction observed at the guideline dose in the present study likely reflects the distinct analytical framework employed here. Unlike prior studies that estimated the total effect of moderate to vigorous activity, which implicitly captures benefits mediated through improved fitness, the joint model explicitly adjusts for Cardio-respiratory fitness.67 Consequently, these estimates represent the conditional effect of MVPA at a given fitness level, isolating the direct benefit of the behaviour itself independent of the physiological fitness reserve.68Despite this consistency in relative effects, our matrix (figure 2B) reveals a subtle absolute gradient, such that individuals with the lowest fitness require approximately 30–50 additional minutes per week compared with those with high fitness to achieve equivalent relative reductions, for example, a 20% reduction requires approximately 370 vs 340 min. This finding highlights the steeper challenge faced by deconditioned populations

Is my math wrong, because that sounds like 10 hours

Who's got 10 hours free? I'm lucky to get 3 or 4 hours a week (maybe 5) at the gym.